I'm not aiming to be unsympathetic or disrespectful - this isn't a binary situation where folks have no choice. I am saying it is unrealistic to scope and build all transit for all needs - e.g. there are no tubes on the Met line and people do City - Amersham, that's a solid hour. But there are other options too - like a change for Marylebone, if you needed that provision.You mention Japan as having routes with longitudinal seating and no toilets. True, but the longest of such routes do have toilets on board, albeit only a limited provision. They also have well maintained toilets within the gateline at pretty much every station, along with super high frequencies so that if someone needs to get off to use the toilet, it won't normally wreck their journey.
I actually agree that not having toilets on these trains is not the end of the world, especially because there are often alternatives available. But you are massively underestimating the number of people who are affected. And worse, you are simply being dismissive and disrespectful of people who aren't as fortunate as you.
In this case, Didcot to London has toilets at both stations and on the fast trains. But people with issues wouldn't be able to take the slower trains for the whole slog, without a plan (going in advance, hopping off somewhere en route etc) - it's not exhaustive and shouldn't be a reason to not operationally extend routes that might make sense for combining. End to end passengers will be few.
Agree that Japan are better for toilet provision in stations - less vandalism, more pride in civics, cleaning etc - we are who we are, and sometimes we don't have the investment, ethics or the behaviour for what other countries can provide. And they do some crazy complex through-running on subway stock - but I imagine very few end to end users - but yes in theory, many 2 hour + journeys on suburban, side seated stock.