2040 is a very long time to wait for a Nuclear SubYea, I had this theory...
So drones now are super cheap.
Drones too don't really need oxygen to breathe. They basically need a battery or some power source, they could get a bit of Tidal power...
TLDR, a low power floating landmine that could exist in the water for months.
A say 30 Billion sub would be a prime target. people would happily make a mine for $500,000 and maybe deploy 20 of them to take one out.... Now could a sub lead a drone fleet, maybe. Defensively though they would be very vulnerable.
People need air, drones dont. Changes the design and threat characteristics entirely. ...
Ukraine are making under water mines which the ruskies can't detect