It's pretty natural not to reserve seats on shinkansen, because you can find seats unless you are travelling at peak hours (and there are trains every 20 minutes or better)
The travel time to and from airport, and the baggage+security easily eats into the 1.5 hour savings. Same day fare on shinkansen remains constant, unlike 30k+ that flights demand.
On shinkansen, you have lots of leg room compared to LCC seats. There's also enough space to move, talk and option to reconfigure the seats for a group of 4 or 6 travelers. There's cell connectivity (and decent wifi onboard) so you don't have to pay through your nose for in flight WiFi. The toilets are spacious. There's dedicated place to talk on the phone. Less noisy and fewer bumps than a flight.
This makes the bullet trains really attractive for business and family travels (with kids). You don't need to plan beforehand and there's less inconvenience compared to flight. Moreover, the cost also balances out if you're traveling to a smaller city with poor air connectivity.
These kind of options actually allow spur of the moment travels over such distances.
I know plenty of people who plan and use bus and flights due to the cost benefit, but also tons of people prefer the hassle free travel on shinkansen
The OP is correct wrt powerful e bikes sharing space with pedestrians and normal bikes.
They are a different beast, heavier and noisier. They have much higher speed limit, and require less effort (some models need no pedal power) to travel. This, alongside the rise of delivery services, encourages people to overspeed (more than 20mph).
15mph is roughly the limit of what makes bicycles safe for mixing with pedestrians, but beyond this speed, they aren't that different from a motorbike in terms of road design considerations.
At least they are better than cars and SUVs