It might, however, save the company a few cents per unit. At the cost of making the device unupgradable -- which lowers your resale value even if you never do it. So they cost you dozens to hundreds of dollars so they can save a few cents. That's customer-hostile.
And, of course, the other reason they do it is to make the device unupgradable, so you have to buy another one sooner. Which is downright malicious.
And because of that the trade off really goes the other way. If you're that tight for space you could put some logic on the part of the logic board beneath the slot and make the logic board smaller, leaving more room for the battery and allowing you to have a thinner battery with the same capacity.
Moreover, "light with good battery life and good performance" doesn't even specify thickness. And quite sensibly so, because that's always been more of a marketing number about who can shave off an extra millimeter than anything that really matters. It's not as if one of them is 7mm thick and the other one is 70mm. And the pointless obsession with trivial differences is how we got this:
https://www.theverge.com/circuitbreaker/2018/5/24/17389220/a...
1. It's infinitely better for the environment. Reducing consumption and reusing working hardware are the most impactful part of the cycle. Recycling is mostly a scam.
2. It's cheaper for you.
- Instead of throwing away the entire product when you run out of storage, you can just upgrade to 2TB drive for about $100 yourself, or pay someone $50 to do it for you.
- Instead of replacing a $1000+ product when some poorly designed $1 component dies, you can get it repaired for $100.
- You might think that paying for a new device is not a big deal, but you should extrapolate these effects across the entire supply chain of everything you consume. How much more are you paying for food when the retailer is wasting money on tech that ends up in a landfill, distribution is more expensive because vehicle manufacturers make maintenance more expensive with proprietary parts only accessible through the dealer, and farms have to wait days or weeks for heavy equipment repair because the manufacturer won't supply them with repair manuals and tools they need to repair their own stuff?
3. This is closely related to #1 and #2, but worth mentioning as a separate point. It provides local jobs for skilled technicians rather than filling landfills and concentrating profits at the companies who are incentivized to design products to have a limited lifespan.
4. As sibling comment mentions, the tradeoffs you mentioned are often a false dichotomy. You can have replaceable batteries without losing water resistance. Replaceable storage drivers in laptops don't add any noticeable weight. Devices don't need to be literally glued together to achieve any goals that end users care about. They do this not because it makes the experience better, but because it's easier to sell you on a new device when repair costs way more than it should, approaching or exceeding replacement cost.