I understand this "offer" from Microsoft, or maybe "strategy", but that's not a replacement for an ideally stable ReactOS.

As somebody else pointed out, security is a primary concern.

Once Windows XP support is phased out, the product is gone.

Installing it is going to be a serious security risk, unless very specific cases, e.g. machines disconnected from internet.

Of course, that's not the only reason; Microsoft will stop selling licenses, then what about installing it in VMs on a Linux host?

An O/S is part of an ecosystem. Once it's phased out, if it's closed source, it's dead unless it's taken over by a third party (see O/S 2).