[9point6]

There's a good chance this might be down to Apple's constantly changing foundations and the fact that a studio's worth of music software can (especially back then) cost a lot of money to upgrade on top of the new hardware requiring the upgrade in the first place. And that's not going into the case where the company that wrote the software has gone bust.

Given you mentioned G4, there's a good chance they had recent memory of having to upgrade a load of stuff to jump from OS9 to OSX. The Intel architecture change required another upgrade for a lot of software, so it probably made a lot of sense to try and eek more value out of the previous round of purchasing before going again.

Hopefully they delayed enough to not get hit rapidly by the changes in Yosemite, Mojave or Catalina which all necessitated upgrades (IIRC, one might have only been some edge cases) or finally the most recent jump to ARM—which still isn't natively supported by a surprising number of DAW or VST vendors.

On Windows I can run any DAW or VST that's been written since about 2006 natively and can even load up a 32-bit DAW if I want to dust off some ancient project with ancient VSTs that never made the jump to 64-bits.