I have a lot of Gen 4 as well as Aiko 3 and Sadie content (and Mavka is becoming a major player as well). To make Genesis useful with my Gen 4 and Aiko 3 runtimes requires fairly expensive outlay for the following plugins:
V4/M4 Body Shapes for Genesis
A3/H3 Body Shapes for Genesis
Texture Converter2: V4/A4 Plugin
Texture Converter2: M4 Plugin
Texture Converter2: A3 Plugin
I think you have the argument slightly twisted around - it isn’t that you need to make Genesis useful with existing runtimes, rather that both Genesis and your existing runtimes will be useful, and that there is additionally a purely optional opportunity to make your existing runtimes usable with Genesis. I think you need to weigh how much of that combine functionality you need out of the box, because you don’t have to have any of it, and, even if you want it, it certainly isn’t the case that you need Texture Converter 2 and you have considerable usability even without GenX.
DS4 on its own will give you use of your Gen 4 skins on Genesis out of the box, and autofit will give you use of most Gen 4 clothing items (normal caveat vs shoes). For earlier items and non-DAZ figures, a few may give acceptable autofits, others can be rigged if you really want them - I’ve done this myself on a few trial items despite never having rigged anything before. Expenditure to get to this level of functionality, zero. A brand new figure for free that can use a big chunk of your existing wardrobe and skins - what’s not to like?
Now I’d say you really want to add the Genesis Evolution Morph bundle to that on general flexibility grounds, that adds $23 to the price, but it’s no different to V4 needing the V4++ morph pack, and you only need to buy it once, not for each base figure as at Gen 3 or 4.
Add GenX ($32) to the mix and all of your Gen 4 character morphs are available for Genesis, you’ll want the Gen 4 figure packs for Genesis to get them to a near 100% match with Genesis (V4/M4=$15, A4/H4/F4/G4=$18), but it’s not absolutely necessary to use them - I just brought V4 Darcy over to Genesis via GenX as part of creating a specific character, and decided she was actually closer to what I wanted without the V4 morph applied rather than with it. Yes, that’s adding up to a big bite of pocket money, but it isn’t an essential to use Genesis, just a nice to have that lets you leverage off the combined value of Genesis and your Gen 4 characters - its one of those things that can be left for the next good sale!
If Gen 3 is your interest, and you want to leverage your Gen 3 characters and wardrobe with Genesis, then you’ll need the Gen 3 Figure packs (V3/M3/A3/H3/SP3/F3=$30), with the same arguments that applied to Gen 4 and the side note that the figure packs extend Autofit to Gen 3 clothing. Now the difference at Gen 3 is you can’t use Gen 3 character skins out of the box (though autofitting a Gen 3 figure to Genesis will give you a semblance of that with some limitations), and some people claim this means buying TC2 and it’s figure plugins is essential, but I disagree, I think it’s possible to find a workably close Gen 4 skin for most Gen 3 characters and I certainly have no intention of buying TC2.
You can spend the money for all the plugins you listed, but people need to understand that those are optional buys, not essential, you can use a big chunk of your existing runtimes out the box with no expenditure whatsoever. You don’t need to make Genesis compatible with your existing runtimes, that is a bonus, you get Genesis’s own functionality for nothing, and your existing runtimes and Gen 3 and 4 figures remain just as usable. The difference from previous new figures is that there is a way to use older generation content with Genesis, some of which comes for free, and some of which comes at a cost.