AT&T was effectively a government-supported monopoly

It was certainly a monopoly. But hardly government supported. The government has issued major sanction against the company no less than three times.

• The Kingsbury Commitment, which forced AT&T to divest its stake in Western Union, stop acquiring competing independent local phone companies, and allow other companies to connect to its long-distance network
• The 1956 Consent Decree, requiring AT&T to stay out of the emerging computer/data processing market and license its transistor patents royalty-free
• The 1984 Bell System Breakup, which formally split the comment company eight ways.

Solar R&D was hardly private. You’re completely excluding things like the U.S. National Science Foundation’s Advanced Solar Energy Research and Development Division.

The NSF has always languished in funding and has historically failed to deliver major breakthroughs, much less any kind of physical rollout, of a public solar/wind energy grid.

If anything, it has operated as an echo chamber of the fossil fuel industry, downplaying the efficiency and ultimate profitability of a renewable grid.

Hardly a subsidy.

And yet, my point is, even they’re still subsidizing solar orders of magnitude more than nuclear!

Show me the solar powered submarine.

Show me the solar powered aircraft carrier.

Show me any meaningful federally contracted project to implement solar power for domestic or international consumption.

At the very least, I can point you towards Georgia's Vogtle 3 and 4. Nothing like that exists in the renewable space.