Get a pro out to tell you if it has been resanded to many times a bid
a fix of issues. But it probably hasnt been sanded to thin.

a contractor i trust said they are too thin, and though i obviosly
dont know about these things, my gut agrees.

If joists are weak, sister on another one, if a few planks are week I
braced
mine underneath with plywood.

thats what DH has been doing - i think the joists are fine.

Old floors were thick, mine are 3/4 oak
with no subfloor and can take many sandings.

I dont think these were 3/4 - i'm almost sure thinner.

You can go over the old
one but you will save thousands by keeping what you have. A weak plank
may have a hidden knot underneath, so support it from underneath. A
good sanding and 3-6 coats, a few repairs, and it will be great.

definitely a thought - we'd rather save money obviously and put it
toward the heating system. if we could *confidently* reinforce the
planks *and* fill and smooth all cracks i'd be happy keeping old
floor. Sounds like not having a subfloor is not a huge deal on its own