Sometimes I wonder if Celestron deliberately engineers this way just so if anything happens to just one part, you have to purchase a whole new telescope to replace it.
That seems a bit unfair.
Celestron has employed the same basic geometry for connecting a fork mount to a wedge for over 40 years. If you have an original orange tube C8 tripod and wedge, you can take a 2018 4/5SE mount and attach it directly to the fork using the three bolts from the 70s scope. You'll probably need longer bolts and spacers to mount a CPC, Evo, or 6/8SE as their bases are wider than the older equatorial forks, but the bolt spacing is correct.
The OTA I use most on my Evo mount isn't the C6 that I bought with it, but rather a 1980 orange tube.
The OTA I use most often on my 4SE mount is a omni C5 (which originally sat on a CG4 GEM mount).
Celestron only changed the tripod-to-wedge linkage when the advent of goto forks allowed for wedge-free operation for visual use in an alt-az configuration.
It is true, however, that there is much less interchangeability among Celestron's GEM mounts. However, this hardly a case of planned obsolescence. Historically, Celestron's GEMs have relied heavily on borrowed components from other manufacturers (Vixen, Losmandy, Chinese Vixen clones) to save design costs.
As others have stated, the fork and GEM tripods have different design requirements.