Where are you seeing signs of a web replacer? Native apps? Or big social media platforms built on the web, that replace the need (for mose users) to 'touch' the base web layers directly?
It would be sad if those bury the web and bury our rich, free open platform paradigm.
If a good decentralised open thing replaced it instead, cool.
It's like tragedy of the commons. Open platforms are always susceptible to EEE (embrace extend extinguish) by design (would like to be proven wrong). And there will always be businesses (people) who when platform is popular enough would like to take over that for profit.
A commons is indeed vulnerable to collapse though bad management. So it won't take care of itself, but needs deliberate effort and coordination to maintain. If we want to keep the open web, can't we put the necessary work in?
One challenge for web-enjoyers is it's not obvious in a consensus-making way where 'weak points' that need help are, and what's an acceptable loss (say: video on the web. We can post raw video on a webhost. It's expensive, so no one does. And most video content is crap, so who cares - let the platforms have that?)
I disagree that open platforms are always susceptible to EEE. In my mind, their openness (or more precisely, the true openness of their protocol(s)) prevents that.
Sure, there's a lot of DRM crap on the WWW now, but there's also still plenty of plaintext. The mainstream going with the EEE crowd is a given (being money people, they have the motive, means, and opportunity to streamline onboarding in a way the openness true believers don't), but the WWW platform remains fundamentally open despite everything thanks to the openness of the protocols it's built on.
A classic example of EEE closer to a protocol takeover is how Google handled XMPP. Yet people still use XMPP. It's just not huge. But open != huge or mainstream. Usually quite the opposite (see above about money people herding the mainstream to their platforms).
A rather more blatant takeover is the more recent hostile takeover of Freenode IRC. In that case the community revolted against the takeover and relocated (some to another IRC server and IIRC some to Matrix)
Did it fulfil original goals beyond expectations? Yes.
Is it going to be replace with something else? Looks like it.
Will that be Xanadu? No.