I've had the same thought and took it even further. The cloud companies are in the business of managing capital, namely datacenters. They are not in the business of designing µarchs; to the extent that they do (ie. Graviton) it is to lower costs. For this reason I belive it is economically favorable for them to jointly create and fund an open µarch. Note thay they are already doing this through a middleman known as the Intel/AMD monopoly. Also note that a similar thing has happened already with Linux. Collectively throwing a few hundred million at the problem (or buying sifive, for example)
could save money in the long run. Although I can't be certain about that.
I hope someone pitches this idea to these companies. I think some people at Google may be thinking in this direction being that they are paying for tape out of open hardware at no cost to the developers and are investing in tooling.