I think what OP meant by "serious airplane" was something that could compete with at least a regional jet for short-distance passenger transport (~50-100 seats with at least 500nm range). Almost all battery-electric aircraft in production or on the drawing board are either proofs-of-concept or geared toward hobby use, flight training, or extremely short-range air taxi services (Harbour Air is a fantastic example of a current company for whom battery-electric aircraft make sense for some commercial missions). They're serious in the sense that some of them will fly well enough to become commercially viable, but not serious in the sense of challenging current turbofan airliners for a significant share of the air transport market.

https://venturi.aero/#highlights claims 50 passengers, 1,000 km, and 40% reduction in operating costs.

The site doesn’t give much more info that that, so I don’t think they’re further than the drawing board, though.

I considered this might be a scam or a “give us money and we’ll see whether this really can be done”, but being a startup with Delft roots (https://en.wikipedia.org/wiki/TU_Delft_Faculty_of_Aerospace_...) gives them some credibility, I think.

Yeah, I have the feeling that they're still very much in the initial design phase, but even so 100% electric doesn't even imply battery-electric (and even if Venturi does plan on doing battery-electric, as their site vaguely implies, there's a good change they're basing their numbers off of an experimental battery chemistry that's being tested to replace lithium ion). Hydrogen fuel cells offer energy densities comparable to liquid fuels, although as mentioned previously they have volumetric issues due to the requirement for high-pressure and/or cryogenic storage of the hydrogen. I think the approach that these guys (https://en.wikipedia.org/wiki/Wright_Electric) are taking is more likely to work out in the near term, since their main problem is finding places in an existing airframe to store a bunch of hydrogen tanks.

I think it would be kind of cool if we all get stuck where we were and couldn't use air travel anymore due to no fossil fuels. Maybe I just like retro stuff too much.

Or restart the zeppelin-age, travel at 150km/h with windows open hanging from a giant hydrogen filled structure (not enough helium to replace all aircraft).