I don't think the argument that the mere existence of clients that work differently ruin the modern features somehow is really that fair (see below). The bouncer argument is kinda fair, but if you also don't like to live in a walled gardens (slack or discord), it limits the options a lot (although there are IRC servers that have integrated bouncers! Matrix is kinda like running your own bouncer again, unless you are ok with a third-party running it for you). I can also accept that there are many more non-modern IRC clients than modern ones that work the way you would expect, so the overall expectation would be biased. And that probably it was too little too late.

But I think you are overthinking it by a lot. If you were to use IRC, you should just use a modern "magical" IRC clients and not worry about what happens in the background (and btw it's not just "could" but "does".. there are clients that do all that already - where you can just drag and drop stuff in and it will magically do the right thing). And I am willing to bet that in other instances you already do operate that way. Unless your mail client is very broken it will send a plain text version of your email along with the html email. Do you worry there too that I am actually just looking at the plain text version of your email and not with the intended html formatting? Or do you worry that the person you are talking to on slack might just be connected via matterircd via IRC (or directly via IRC back before slack did the bait 'n switch) and not see any of your snippets, images, etc.? Which btw. I am totally doing despite how much it butchers everything - I just cannot stand that UI (and neither can my rather old laptop).