Sony tried this and failed with the PS3 store but I think Microsoft is prepared for the backlash and will go through with it. The reason to close the store is almost certainly payment security. It's an ancient platform and allowing purchases on it is probably a security nightmare. They won't be taking down the ability to redownload stuff and a lot of the games have migrated to the Xbox One/Series store, which is presumably built to modern spec and easier to keep current from a security standpoint because it is, I believe, HTML based instead of custom storefront software.
Personally I'm a little sad about this but I can't say I'm overly affected. I have bought a couple things on the 360 store this year but they were minor and most of my favorite 360 games are backwards compatible or only disc based anyway. The store was open for 18 years, which is a pretty good run, and the games will be downloadable even after this. From a preservation perspective it's pretty bad and Microsoft has talked a big game about that, but at the end of the day it's a digital storefront that likely isn't making any money.
I hope there are some big blowout sales before it goes, and maybe a few games moved over to backwards compatibility.