Sotomayor- yes, she probably should retire by 2024
Kagan- no, unless she knows she is very sick for some private reason
This is the probably what I would advise where I a partisan democrat. The Senate map in 2024 is brutal for them and they could easily lose it even if they win the presidency by a good margin.
I thought Democrats already lost those seats in 2018.
They lost four (and gaines two) that year, though Manchin (West Virginia), Tester (Montana) and Brown (Ohio) are at high risks. Arizona potentially as well, and Wisconsin is also an option (I don't think the others up in 2024 are competitive). On the flip side, Democrats have almost nothing to gain other than Texas against Cruz, which is a heavy lift. And Florida isn't going to happen. Biden could win reelection, even take back the House and lose the senate regardless. Being at 51 (assuming Warnock wins) would give them a small window to hold on, but I'd much rather be at 52 or 53 now.