so there is no consequence for creating a community and abandoning it

You could also argue there's almost no consequence to farming domain names as their cost is change money to many (I'm not up-to-date but a .com should be like $20 or less per year).

If I ran a bot that automatically claimed the names of the largest Reddit subs on your instance

That falls under spam, which is technically easy to block. Again, you are picking extreme examples. If you need them that means that your point was without merit from the start.

would be solved if admins manually review

The admins are volunteers. I don't think you realize how unreasonable your demand is.

I reiterate that the best and healthiest thing to do is instead of waiting for a community to show signs of inactivity to take their name away, that you instead move on and give up on that name. Be creative and find another name for your new community.