ASIC-Resistant Proof of Work
The downsides outweigh the upsides: as you mentioned this would require a hard fork, and would leave everyone (not just mining farms) who owns a Bitcoin miner with a very expensive paperweight, and as such it's unlikely a large enough portion of the community is willing to make the change for it to be successful. It would require throwing away a significant amount of infrastructure.
I suppose one day if it were to become a problem where Bitcoin miners are colluding to try and rewrite the blockchain or censor transactions, then we could reevaluate this. However, mining already comes with such razor-thin margins that it is extremely unlikely miners will be throwing away their profits to do this: they would all have to agree to work against their own best interests.