It doesn't matter to them if no one has ATSC 3.0 equipment. They can ship a little ATSC 3.0 dongle and cheap antenna to people who subscribe to their DTC RSN scheme, similar to what Directv did with the LCC.
Since they'd be encrypting the RSN channel(s) a TV with an ATSC 3.0 input wouldn't be useful anyway.
I'm not sure where you came up with this cockamamie ATSC 3.0 scheme but that is nowhere in the conversation with the RSNs. The number of subscribers that would be willing to jump through all those hoops is tiny, not to mention how long away we are from ATSC 3.0 being widespread in any form. Also the arrangements with the MVPDs would never, in a million years, let them sell the linear channel via ATSC without the MVPDs being able to drop RSNs from the lowest tiers, so trying some ATSC scheme would bankrupt Bally's immediately
At the outset, Sinclair insinuated they already had the rights to do what they wanted with a direct-to-consumer product. Either they really believed this, incorrectly, or it was a PR ploy. I suspect they thought it was true and simply did not read the contracts with the teams before they bought the RSNs from Disney. When MLB started pushing back, Sinclair changed their tune
Now, what Sinclair is doing, and has done already, is working to re-negotiate with each team as the contracts expire. They have negotiated direct-to-consumer streaming rights with four MLB teams so far, and I think they have DTC streaming rights with most if not all of the NBA teams they carry. The issue now, though, is I'd be real money that the MLB owners will circle the wagons and not sell any more of these rights. Sinclair got in under the radar before it was clear what they wanted to do, but anyone with a brain can see that MLB will just want to expand MLB.tv to the point where blackouts don't exist. It already prints money and is a very good product
Sinclair wants to take RSNs in their current model direct to consumer. They want people to have to buy the RSN, not the sport. So, to get the Braves, you're also going to be paying for the Hawks and the hockey teams from Nashville and Charlotte. MLB wants to sell you just baseball, which is clearly the way the market wants it to work.
Sinclair has a little bit of time before their mountain of debt is due, but they have a TON of debt built up just to buy the RSNs in the first place. The sports leagues might just wait them out and let them go bankrupt, and then they can buy the rights back at pennies on the dollar