I agree with you in principle, but one of the problems is that some exhibitions are so popular that you really have to book weeks in advance if you want to get in.
Case in point: A friend suggested the biggest ever Yayoi Kusama exhibition in Manchester[0], which I relayed to my girlfriend because we would be in the city 3 weeks later.
She sent me a frantic sms Get Tickets Now! since the exhibition was sold out weeks in advance.
And that's an issue you see with a lot of popular exhibitions. You can't just walk into a David Hockney exhibition at Centre Pompidou on a whim, because the time slots are sold out for weeks in advance.
That may not be fair and super inconvenient if you don't book far in advance. But popular exhibitions in big cities are so overwhelmed by visitors that there's really no other way than selling time slots well in advance.
The Kusama exhibition was totally worth it. But we wouldn't have stood a chance if we hadn't organized the tickets weeks before the visit.
I think the insistence on smart phones for basic services is much worse than asking the public to organize a museum visit well in advance if the exhibition is super popular.
[0] https://factoryinternational.org/whats-on/yayoi-kusama-you-m...