As said before there is mobile 4G\5G widely available.
Yes, but not everyone will have roaming. When in the USA, I've been with people who didn't activate roaming and just relied on Wi-Fi because of the cost. Few plans include non-EU roaming (I have it on my legacy EE plan) too - so think of tourists visiting from outside the EU here.
Plus some, not all, roaming agreements are limited to just one operator and if that roaming partner is Telefonica or Hutchison, chances are you'll be trying to use O2 or Three on the train, which can be a
very variable experience.
Oh and not all networks offer 5G roaming either.
There are many reasons NOT to use Wi-Fi (I've given many, plus others have regarding security) but also many reasons TO use it. Let people have the choice.