How is it sinking, though? It's not getting improvements, but that's a very different thing.
It hasn't received major new features in a decade. It's currently owned by the Windows team (not the .NET team) and they have staffed it with a skeleton crew. They don't even have bandwidth to review most community PRs.
Even the .NET team doesn't get responses to issues they raise on the WPF repo: https://github.com/dotnet/wpf/issues/3811
Adding to all that, WPF has a massive dependency on C++/CLI which means it cannot take advantage of many new .NET features until that is rewritten. And from what I can tell (from the outside) that will never happen.
(but yes, true, WPF still works and it gets enough support to keep it limping along)
Editing IDL files without VS tooling just like in the good old COM days pre-.NET, lovely.
But anyway, my point is that it was hardly "abandoned at birth", given that WPF was originally released with .NET 3.0 in 2006, while the first version of VS to get WPF UI was VS 2010 - and more was rewritten since then. Even if no new VS components are ever going to be written in it, maintaining the existing ones makes it necessary to maintain WPF to some bare minimum standard.
(The same goes for WinForms and other UI stacks that VS still uses in places.)