Still waiting for a good answer to this one. When someone does successfully counter it, I imagine the rebuttal will involve Chinese pictograms or something else that has worked well for a while but isn't likely to survive the next thousand years.
Egyptian hieroglyphs were in use for about 3600 years, and as other commenters pointed out, Egypt stopped using them because they were conquered by Rome, which then imposed Christianity — which banned a different kind of "icon", killing the hieroglyphs as a side effect. Also, far from being "icons" in the UI sense, Egyptian hieroglyphs were primarily phonetic, although they did have ideographic components — less so than Chinese characters (which are logographic, but only about 4% of them are pictograms) but more so than the Latin alphabet.
The Latin alphabet, which is a modified subset of the Egyptian hieroglyphs (via the proto-Sinaitic and Phoenician abjads and the Greek, Etruscan, and Old Italic alphabets) has been in use for 2100 years, about 1500 years less than the hieroglyphs.
If we're still using the Latin alphabet around AD 3400, your argument from observed adoption that alphabets work better than pictograms would make sense — if hieroglyphs were pictograms, which they mostly aren't.
Every invention of writing except the khipu started out pictographic: Chinese characters (which have been in use since the Shang dynasty a bit over 3000 years ago, contrary to what you seem to think), Sumerian cuneiform, Mayan hieroglyphs, and Egyptian hieroglyphs all have clear pictographic origins. But all of them developed phonetic components in order to expand the range of language that could be written, and those phonetic components came to dominate the script almost immediately, to the point that none of them have an identifiable pictographic-only period in archaeology. In fact, as the numerical nature of the decoded khipus and much of the early Sumerian tablets suggests, logograms for abstract concepts such as numbers may have been in use as early as pictograms or even earlier. So, the actual history supports your idea that systems relying entirely on pictograms have very limited applicability, even though it has nothing in common with the history you imagined supported that idea.
On the gripping hand, maybe computers are not the same medium as clay tablets and printed paper, and so maybe past experience is not entirely applicable.
In hiragana you have 48 letters for sounds which in most cases are a consonant + vowel. Hence Japanese written in Hiragana or Katakana is shorter than it would be if it was written with a western alphabet. Japanese is probably not the language with shortest words, but neither are they exceptionally long.
(b) So why'd the Egyptians stop using them? What makes them right for today's needs?
Egyptian hieroglyphs, in their ornate form, clearly depict various items like reeds, birds, arms, and snakes. But those glyphs don't actually refer to the reeds, birds, arms, and snakes they show; rather, the ancient Egyptian writing system is largely alphabetic. Their formal letters were just much prettier than ours.
It wasn't a voluntary act, trust me and with the rise of the emojis as a medium of communication, I can say that we or the ancient Egyptians had it right all along in using hieroglyphs.
I'm out. This thread is 200TB of crazy on a 10TB drive.
Also, you're EXTREMELY overestimating the individual agency of the ancient Egyptian in this matter. Most of the citizens of ancient Egypt were illiterate and only a certain class or caste had the privilege to be able to read and write but the rest of the population were completely oblivious to those icons and therefore your argument is just baseless.
And in fact that's why they're HIEROglyphics, they were the priestly writings. (compare to demotic, as also seen on the Rosetta stone).
English is the primary programming language, not because a team of dedicated Anglo-Saxons are pointing rifles as the head of brown people, but because American trade provides the most profit motive between 1974-2008.
A "primary methodology" only applies in "most" cases, not all of them.
Lots of languages have perished for outside forces other than trade -- the Hebrew wasn't abandoned because a better trade language came around, but because Israel was conquered and the locals scattered. Other languages were persecuted by decree. Others died because their native speakers were eliminated. And several other ways (e.g. Mussolini tried to smooth out local dialects of italian using the powers of government plus radio and early cinema).
Religion can and does preserve memes and linguistics, but only trade can scale it out.
If it was just for trade, language learning would be relegated to merchants in those countries and few others.
Government, bureaucracy and occupation matters more than trade in this regard. That's how latin become the norm in a large area during the roman empire -- and not because everybody in those regions traded directly with Romans or couldn't agree on pricing otherwise.
Same e.g. for French -- it's not because of trade they got big as a language in the 18-early 20th centuries, but because the french run a big colonial empire.
And English, beyond trade, were the language of the big British colonial empire, and afterwards of the dominant culturally power that was America (Hollywood, rock, pop culture etc.).
That would make sense if you had many debates with the parent and he always blamed unrelated stuff to imperialism.
In this case he talks about a specific historical example, where we have a specific historical account, which happens to agree with the parent.
They didn't just go out of style, they were phazed out when the country was invaded, along with other aspects of the local culture. For centuries after Alexander, in Egypt you wouldn't get promoted to the higher ranks unless you were Greek (or Greek speaking) for example, and the local population was kept as second rate citizens. Then came the Romans, then the Arabs, ...
But clearly you have a specific self-made explanation, that they just "couldn't work" and were abandoned for that, history be damned.