Tiny(anything) is okay if you need just rudimentary access to a computer for testing...it's not real great for the long haul. lots of stuff you may end up needing for the long haul ends up being deleted or disabled to improve the load time...then you start futzing around with the OS getting things going and you end up with basically a full OS load after it's all said and done...
that said, lots of people use tinyxp or tiny7 for their arcade with no issues.
windows 7 licences are a dime a dozen. I literally peeled an HP key off a busted all in one computer and used my windows 7 home premium 64 disk to install. using that key... on a computer I built to use at work...not an HP machine... with no problem...then I "upgraded" to windows 10. so yeah free.
if you have an old busted laptop or something with a windows key, you can legally transfer the key onto a new machine. you may have to phone and activate (automated system 24/7 access) but you are legally allowed to do it.
there is a software swap subreddit (which will remain unnamed here) in which you can purchase windows keys for like 10 or 20 bucks each...90% of these keys come from corporate machines in which they use their volume licence and image 100's of computers...so the licence sticker that comes with the dell or HP computer goes "unused".
going legit is good in the sense that things stay updated and you don't have to worry about your installation becoming deactivated.
besides, with SSD prices these days, it's pretty easy to get a 60 buck 128gb drive and have a FULL LEGIT OS load faster than a stripped down tinyOS load on a platter drive.
What? I wasn't worried about a key, I own them all from DOS 3.1 up to Windows 10. I still have 4 keys to Vista Ultimate but it's complete overkill. The point of running "tiny" is to not only streamline the OS for specific use but offers near instant boot of the shell, that being GameEx for me. Not much point in access to servers, printing, search...etc if I'm just launching MAME games is there?
My steam gaming machine doesn't even launch Explorer, I use Steam as the shell, it's as streamlined as an Xbox and boots way faster than my Windows 10 machine. Using a Tiny 7 for that, streamlined just with the files necessary for gaming but is still 4 gigs because some file removals from WINSXS causes issues during the processing resulting in boot issues.
I really don't need anything but XP to run emulators and since the SSD drive I'll be using is only 64 gigs, TinyXP would make more sense here. Just not sure how it's well its running on modern hardware these days. A tiny 7 is easy enough but it still takes up over a gig more space than a basic XP custom iso.
And for another thing. If your intention is to use it in an arcade cabinet, and you won't have it hooked up to the internet, then no reason to bother updating it any more then you need it to perform best. No need for anti virus, windows defender, ANYTHING used for internet or wireless transfer.. ditch it.
Yes, that's the part of the whole point.

. The support from MS thing was funny though. However money is not an issue for me, I own them all and quite a few keys. I used to build PC's for people.