Let's say you have ~65% equity with top pair versus your opponent's calling range. You're playing an average player. In this case, you have J9o, on a Jh4s2s board. Let's say we're SB vs BB and we minraised pre. You're OOP and bet full pot and your opponent calls. Turn is a random card (that doesn't change our hand ranking).
Do you have at least 40% equity here?

Bet the turn to get value from flush draws, straight draws, overcards, and middle pairs (not really many worse jacks that will be in his range that often). If the river doesn't complete a draw, bluffcatch or value bet small.

I would say it depends on what villains range actually is. But I would say that most cards would increase our equity above 65% since most cards do not complete draws. Of the hands that we beat, draws are going to have the best equity against our hand right? Like TT has horrible equity against us but a flush draw has 9 outs x2. On the turn however, the majority of the time the flush draws equity is basically slashed in half. Then again, other hands will pick up some equity on the turn and still be behind us but I don't think that effects things too much because that increase in equity is usually small.

If we can assume that we have 65% equity on the flop, then why can't we assign a range.

Another thing to consider is villain's calling range. If he is not calling with his entire range, then its easy to see that our equity can drop way down if say villain only calls with hands that beat us.

If you have 65% equity on villain’s call with you holding J9o and the stated flop, according to Poker Stove villain has a top 22% hand. I checked various turn cards that are likely to improve villain’s hand and the only one that reduces your equity below 40% is the ace of spades. Therefore, it is highly unlikely that your equity after the turn is less than 40%