It tended to work badly when more than half the people on the flight had opted for it.
Though looks like easyjet have restricted its availability now.
easyJet have always restricted the availability of it for purchase - I think it was 12 on a 319 and a few more on a 320. Nowadays it's based on seating - you get it if you buy the front few rows or exit row, which is a bit more than 12 but still not a huge proportion of the aircraft. TBH, its role has now reduced anyway - it used to be about seat selection, now it's more about luggage space particularly if in the exit row and not able to put stuff under the seat.
You tended (and still do on primarily business flights) to get a larger number of people because they are easyJet Plus cardholders, whose numbers are not restricted. This has been controlled a bit by increasing the cost of the card. Some employers will cover the cost, some won't, but it's still reasonably priced enough to be a personal purchase to make business travel less fraught.
Ryanair, OTOH, will sell it to the entire plane, but that's Ryanair for you.
Anyway, this is more one for the aviation thread
FWIW, back to Uber, it wouldn't matter if everyone opted for the uplift, as unlike seats on an aircraft the number of taxis on the road is potentially elastic - some drivers who are sitting at home not working may decide to go and do some driving for a bit if there are higher fares on offer, and more widely may plan their day around times when higher fares are likely to be on offer. If everyone opted for 30% uplift, it'd still bring more cars out onto the road as it does now - you'd lose the prioritisation effect, but you'd still get the other effect.