You have a good point with this question, but is hard to implement and will never be an exact estimation. To estimate fees before the tx is sent, the wallet need to do a heavy probing. And probing over LN is really bad, is fucking the whole routes.

Maybe when something like "valve system" Rene Pickhardt proposed in his latest study (and also I was proposing this even before in this experiement) we could not have a good estimation and also without damaging too much LN path finding and routes.

But yes, I agree with your point, user need to have at least a close estimation about how much will be the fees. Nowadays we have too many predators that only want "profits" from running a LN node...