I've heard of a lot of use cases, but this is a new one to me.
Then maybe you shouldn't assume what 99% of users actually need of testnet, any mining related project will need to mine blocks to test that their software can correctly construct a full block, deal with mempool, hash it correctly, and propagate to other nodes.
Then build it on testnet and don't get upset when someone beats you at the set rules...
I am not upset for myself, I only spent a few months on mining development back when I first started mining and had lotsa free time, I no longer do that but I feel the frustration mining devs endure because some greedy folks want to abuse the rules to get all the coins to sell them on "exchanges".
Actually, even some devs are greedy and selfish, I recall when testnet had a diff spike, was talking about on a telegram group tell another dev said he had to point his S9s to get a few fast blocks.
I can rekt testnet4 single handed, I can spare a few PHs and point it there just to prove a point, in fact, many other fellow miners can do just that, but then what? you are not going to make testnet4 any better, so all we trying to do now is come up with solutions on code/consensus level.
Obviously, many folks do not think there is an issue with the current setup, that is fine if you don't see an issue and your business is doing well, others who are affected will keep seeking for solutions, its the reason why we have testnet4 and will have testnet5 and so on.