@roytam1's Pale Moon fork does not target Windows XP exclusively; it can run very well on Windows OSes past XP, i.e. Vista, 7, 8, 8.1, 10; "our" maintainer has, among other things, extended upstream support "below" the officially sanctioned Windows OSes, to include Vista/XP/2k (probably with "Extended Kernel" installed); I see no reason to remove support for OSes other than XP (this would be plainly selfish coming from XP users), since our UXP fork has already diverged enough from upstream to be a separate platform...
I am aware of people (several of them here on MSFN) on Win7+ using New Moon 28 and/or MyPal 28 because they prefer them to the upstream offering... Divergence is more pronounced on the Serpent 52 fork, e.g. it has retained WebExtentions support, thus a person on, say, Win8.1 can't use official Basilisk if said person wants a "legacy-style" browser with additional WE support; e.g., recent discussion about WEs restoring "classic" Youtube layout comes to mind...
Denying people the ability to appreciate @roytam1's offerings on OSes other than Windows XP is an OS-discriminatory stance akin to the one exhibited by upstream, denying their applications to Vista/XP users ; and I stand to be corrected, but Win8-specific codepaths in the platform won't even load when the application is launched under XP, so I wouldn't call "Start Menu Tiles" support in the platform/app code a "feature" that has "crept"...
The majority of people here that have stayed on Win<7 are tied to some older hardware which, for a wide variety of reasons, don't want/can't make parts with... Each one's (older) hardware combination (motherboard, CPU, GPU, etc.) is probably unique, so it might be unwise to generalise on overall performance going just by one's own particular setup(s)... Thankfully, Roy tries to maintain a wider variety of forked browsers to accommodate most older setups, the key here is what works best for "you" (your H/W, S/W, workflow, etc.).
I'll refrain from commenting on your "security" views, I said recently that this is a (mostly) democratic community, thus all opinions are welcome/heard (to the point they don't violate forum rules); FWIW, we have here proponents of both extremes on the subject of "security"...
Best regards