If you're pregnant and want my seat then just ask me - I'll always happily give it up.
The lesson here for me, as a man who will never face the problem of not finding a seat when I'm pregnant, is that there are huge problem spaces that I have no insight in to.
This is an minimum viable product for a problem that I don't understand or have knowledge of. It won't work in every situation, and there are some huge problems to resolve in getting enough traction for it to be genuinely useful, but if this problem exists (and I'm willing to accept that it does) then I see this as a positive step forwards.
This is a really common situation with most startup ideas. If you ask people who have no information about the problem what they think you'll get feedback that isn't worth anything. This is one of the reasons why getting good feedback is hard.
One is whether the existing solution (asking) works well. I wouldn't know, and I'm fully prepared to believe that it doesn't. The old "breaching experiments" about asking for someone's subway seat come to mind - asking for a seat while pregnant is very different from asking for one with no reason, but there's still a strong impulse against asking strangers for concessions. Add in the possibility of a hostile response, and I certainly won't be making fun of anyone for "not just talking".
But the second question is whether the solution works any better. The requirements for that are "people download the app, keep location and bluetooth on, then notice the alert and respond". That, simply put, isn't going to happen. You don't have to tell me what the product is - just knowing it's a two-sided 'market', where one side is "do all of these tasks without a reward", I can tell you that adoption will never be high enough to count on the app (unless someone enforces its use).
I don't object to someone making this app, though I'm a bit uncomfortable with them charging for a product which will work approximately never (even for charity). But it feels more like an art/awareness project than an actual tool - it's motivation to consider the problem, but I can't see any road for it to be a solution.
Pregnant women may well pay for the app, but I think the only reason others would pay that much, for an app that will probably never actually alert them, would be to donate money to charity.