For the reason of all the good things that you said, I still think the BlackTip is a great first scooter.
The investment is small (in DPV $$$ terms).
The BT is very capable. For people just doing wreck and reef diving, they may never "outgrow" a BlackTip. Especially a BT with a 20Ah battery pack. It goes plenty deep. and you don't generally need really long run-time on a wreck dive. But, if you are diving a CCR on reef dives, yeah, you might eventually, maybe, get to the point of wanting longer runtime (though, a 3 hour reef dive is a pretty long dive, and are you really going to be on the trigger the whole time? So maybe you could really do 4 or more hours?)
For when you first start out cave diving with a DPV, do you really need more run time than a BT w/custom battery gives? I mean, with just 2 x Dewalt 12s, it has a 3 mile range. That means 5280' of penetration, 5280 out, and the same left in reserve. Does someone new to cave DPV really need more than that?
And with a 20Ah pack, your Third is even further penetration.
It seems to me like a BT is great for a first cave DPV. And when you are ready to do more penetration than that, then the BT becomes a good tow scooter to have when you buy your Genesis (or whatever). I think very few cave divers ever go far enough in that a BT (used as a backup/tow scooter) would not be able to get them back out. Some do, but VERY few.
ANY scooter could break, so you always have to plan your dive to accommodate a DPV that dies. That's true whether you're using a BT or a Genesis.
So, when your DPV does die in a cave, well, it's an inconvenience, right? Not an emergency.
Which suggests to me that, unless a BlackTip is something that is going to die often, then it's probably reliable enough for the range it offers. Yeah? And, while I don't like the reliability of any PTB-based scooter, I don't think the BT is so unreliable that it isn't suitable for cave DPV dives in its range.
Now, I could be all wrong. I am trained for cave diving, but not cave DPV use, yet. So, maybe I'm just wrong.
This all seems a lot like what I tell people about lights. You don't need a big canister light as your first light. Good a good cordless. When you get to the point of needing a big can light, then that cordless light will be a great backup, which you will need to have anyway.