Sounds like someone has abandonment issues lol
I jest.
While I can understand that you want some sort of protection/guarantee to ensure the longevity of your site, I don't think requesting that devs give up their rights to their work is the way to go. To me, it's pretty ludicrous and seems to be demanding way too much for your money.
I think if you're that worried about add-on updates, support, etc, then you'd be better off paying for custom work where you can decide everything and own the rights to it all and that way, if one dev lets you down, you can pay for another to take over/maintain it. It might cost you more money, but like the ol' saying goes...."you get what you pay for". If you're paying $5-$60 for a license to use the code "as is", then that's exactly what you get. You can't expect a lifetime guarantee of updates, support and a possible exchange of rights to all be included for $5-$60.
I've created an add-on that I think a lot of people will find useful, but there's quite a few threads similar to this one, that just put me off from selling it on here. I think a lot of people expect too much for their money. In my mind, if you're paying for a license to use something, then that's exactly what you have, you don't own it, the creator or the rights to it. You certainly don't have the right to be telling/demanding/requesting what they do with their work if they decide that they no longer have any interest in maintaining it.
While considering selling my add-on here, I have written a product description that includes important information, I think I might need to add this to it...
"If I create it, I own it. If I leave it unmaintained, I own it. If I abandon it, I still own it."
lol