Let's be more clear about this.
The question is always rent or buy if you're not using the surplus capacity of some other owner's GPU farm. Six years down the road your RTX 3090 is going to look as dated as the GTX 760 I'm currently running, and you'll have spent an average of $300 per year on the acquisition cost alone.
NVidia's newest CPU+GPU line "Grace" is data-center only. It won't be the end-of-the-road for consumer-grade GPUs for a while, just as chip makers still make enthusiast-level CPUs. Their price however is going through the roof as there will be stiff competition for dwindling supply. GPU volume is shifting to mobile and cloud as AI is ramping up to become the dominant application for GPUs. Sooner or later the rent-or-buy question will be about datacenter-based GPU, whether it's in the public cloud or some organization's compute resource.
The largest contributors to PG on GPU subprojects are either already renting cloud-based GPUs or they are relying on fleets of PCs set up for business or education. The fleets will evaporate in the next round of hardware refresh as computing moves to the cloud at scale simply because of datacenter economics.
So at some point PrimeGrid members who want to own GPU capacity will have to decide whether to spend ever more money on a top-of-the-line consumer-grade GPU, or a buy a share of a datacenter-based resource. The question hasn't been broached before, but this is certain to happen within the expected lifetime of many GPU subprojects.