Languages change and always have, people noting this is as old as written record about language. People have also always been complaining about it, and ascribing to it some meaning about the increasing degeneracy of the youth or the decline of society or whatever.
The language obviously continues to work fine for the needs people have of it, as it always has. You don't need to like the changes, but you should understand that that is an aesthetic judgement and nothing else. It doesn't indicate or represent anything "going down" or a cultural loss of any kind.
With this in mind I find your purely relativistic idea that all languages are equal, or more importantly that we should treat them as all equal, as somewhat misguided, and in many ways supremacist (it is easy for the culturally dominant, in this case the english, to wave hands and act like everyone else is irrational for trying to protect their culture, when it is implicitly understood if they don't then English will swallow it whole).
Old English was a way more complex language, akin to German and had a different form for each person. For instance, to shine was "sċīnan", and the present tense was
- ic sċīne - þū sċīnst - hē sċīnþ ...
Nowadays, it's just "I shine, you shine, he shines", without no distinction except the third person's 's'. It's a vastly simpler language, and yet, it allows people to convey concepts as well as any other language.
A thing I find ironic is that in certain languages that do thoroughly conjugate verbs, there's a way to mock those foreigners who don't know how to use verbs properly. It often happens that those who lack the knowledge of the proper inflected forms use the infinitive everywhere as a stopgap - for instance, one may say in broken Italian "io avere freddo" instead of the proper "io ho freddo", and every Italian speaker would understand it perfectly, albeit a native speaker would probably scoff it off as a blatant sign of a lack of linguistic knowledge.
That's precisely what English went through during the transition from Old to Middle English. People started speaking "broken" English and they kept doing that, and after a while it wasn't broken anymore. Subjunctive tense? That's basically dead in modern English too.
The point I'm trying to convey is, what you consider as the rules nowadays come straight from the broken babblings of the illiterates of yesteryear. If those Roman scholars that were complaining about the broken Latin spoken by their contemporaries had their way, the Romance languages would have never existed. Ironically, the moment Europe dropped Latin and started adopting the "vulgar" actually languages spoken by the people is widely considered the moment Europe truly started blooming again, and a crucial first step towards the literacy of the masses.
The whole purpose of any language is facilitate communication between people. I would argue that the quality and usefulness of English has increased over time, as it would be very difficult to use English from even 100 years ago in conversation today, as it would lack the words to express every day topics.