Indeed, in what concerns Windows itself, COM has been the cornerstone of Windows APIs since Vista.
If only they would improve the tooling, instead of like I am doing COM in Windows 95 with Visual C++ 5 kind of experience, because every time there is some project to improve the experience it gets killed after a while, and we're back to yet another C++ framework with the MIDL command line compiler.
I think a combination of Office Scripts with maybe Power Automate and perhaps some kind of browser extension? I don't think all of COM will be available in the web applications. They haven't bothered implementing COM in Outlook, so Excel and Word will probably end up becoming web applications too.
I am really curious about the conversations that will be had if or when they seriously propose to any of their banking and finance customers that they Webify excel for windows.
(Though, going on recollection from past Q&As the excel devs have done, a more fundamental issue is that few people working they fully understand how COM works, and they are getting fewer)
The only issue Microsoft has to worry about is legacy COM add-ins for Office products. The 'modern' method of creating add-ins has been around for awhile and has the benefit of also working on macOS and the web, plus not having an add-in that can crash the particular Office product, doesn't require an install on Windows, more secure, etc. etc... There are benefits to Microsoft and to end users. Migrating complex COM add-ins to modern add-ins is still a big lift.
The app itself (Outlook) dropping COM support is simply given it no longer needs COM support for add-ins, it can drop COM itself.
ERP add-ins for Excel are the main reason why I still use Windows at the office. Side note: the webified version of Outlook's new roomfinder is awful; it's slow, doesn't work correctly, and doesn't respect dark mode (but this is a larger failure on the Windows side in general).
Note: this is in the context of 'programmatically extending Office/M365', I don't think the OP was referring to COM in general.
COM is at the base of WinRT, which is pretty much _not_ deprecated.