So I just completed building a fantastic Win XP retro gaming machine to play older stand alone games and it seems like older games just won't be available for me to purchase and run now that Steam and GOG are going away from Windows XP. I don't want to play old games with the latest hardware. They just won't run correctly.

What to do ???

Some games still run on XP though. You just can't install them with Galaxy, only offline installers. Maybe look for mods and patches to make the game work? (I could be wrong on all of this though)

Try to find original copies or ISOs online to burn to disc, maybe? I thought XP was like the last OS to play many 95 and 98-era games, bridging the "old" and "new". Some GOG games you can extract the relevant contents and burn to disc for use on original hardware.

You might have to use 3rd-party tools to extract GOG offline installer.
innoextract works very well.

"Just won't run correctly". Please define. Whilst I am not the greatest fan of win10, I have yet to experience any issues with games old or new, which a bit of tweaking or a mod or two wouldn't fix - although I haven't been through all the thousands of discs I have as yet. Sure there are some with 16bit installers and things but you can get around that with virtual machines, heck you could even play on a virtual machine.
That being said, I do have a separate machine with a 32bit is - win7, for those instances where 64bit isn't available - call of Juarez 1&2 for instance will not run on 64bit, no workaround as yet.
So I would suggest having a main win10 machine - latest, fully up to date for the 90% and then get patches, mods etc. Or use a virtual machine for the odd bit.

The point of the older classics on GOG is so that they would run on modern hardware and OSes. The main target at this point is clearly Windows 10, I don't think GOG would release another game which doesn't run on Windows 10 (but required Windows 7, XP etc. instead, or older hardware).

XP hasn't been supported by Microsoft for many years, and even Windows 7 support is ending after one year or so IIRC. It is unrealistic to expect GOG keep supporting an OS that hasn't been supported by its makers for years.

I personally am still using Windows 7/64bit on my main gaming PC, and on the side Linux Mint (but I don't play much on the latter, yet). I will "migrate" to Windows 10 at home when I buy a new PC at some point, it will have Windows 10 preloaded.

And yeah I also have a couple of XP retro PCs around, to play some older (retail) games that don't work fine on Windows 7.

Let me pop in here..

I notice GOG has Descent I and II here. Those are WinXP games.

If I get them from GOG they will play on XP I am assuming right?

GOG used to provide patched executable and/or game data.
It is known that some games no longer run on Windows XP properly.

GOG no longer support Windwos XP officially.

I think D2 was Win95. Also there was the "Definitive Collection" which may have updated some of the graphics, IIRC. Then there in the 3rd party programs that use the D1/D2 resources (HOG) with a bunch of features.

I'm just assuming that GOG would be selling the original games, like if you found the retail version in a pawn shop.

As usual no company is going to care that you can't run the product you bought on the OS it was designed for or even the latest OS (unless someone complains loud enough on twitter), they already have your money. It will be up to the community as to make sure the games still work. Use cracks, wrappers (SmartSteamEmu works well) or emulators.

DOSBox officially supports Windows 95+
ScummVM officially supports Windows XP+
Use the list of DRM free Steam games
Browse the cs.rin.ru forums
Look for ports for the games although in some cases you may have to use older versions of the ports.
Notify the devs of the software to remove the DRM and/or that Windows XP support is broken. Most of the time it's a simple fix.

As an owner of both games here, I can verify that they are the base DOS versions bundled with the DOSBox application in order to make them run out of the box on modern systems. There is nary a Windows version in sight, nor any of the expansions from the Definitive Collection (Levels of the World, Vertigo).
Some do, but not all. Unfortunately, there is no way to be 100% sure which do without buying it/trusting a forum member's word.

And in some cases, GOG remove original files from all owner's Library.
Only the brand new installers for modern system are downloadable.

I have some xp gaming machines. Never had a problem.

Classic games on GOG run fine for the most part on modern hardware. What you should be doing is buying up the CDs and DVDs that games were released on. These installers were not made for "old school" hard/software.