SIM cards were never swappable for me. They were locked to a phone. Changing to a new handset meant going to the carriers store or having them ship me one. I welcome the ease of portability that eSIM brings.
In France, (many/all?) phones bought from the carriers (with a subscription with some time commitment) were (are?) simlocked, meaning that they could only accept SIMs from these carriers. But carriers are also forced by law to let you sim-unlock your phone for free after a few months. So yes, pretty seamless and common indeed. Everybody expects to be able to switch carriers and keep their phones.
Not an inherently American perspective. I've been able to swap SIM cards between devices in the US using both large and small carriers since the 90s in the US.
In France for example SIM cards where a thing since cellphones exist, however I've lived in Japan and US in the 2000s and in both countries there was no SIM, you had to go through your carrier to change phones and they would setup your phone in the shop.
I'm on docomo and I only have a SIM from them, I brought my own iPhone from abroad. When I turned my iPhone in to Bic Camera to have the display swapped, I could just swap my SIM into a spare old phone quickly and it worked seamlessly.
Oh that's good, I went into one of their branches and they told me it wasn't possible and I'd have to buy a phone from them. Probably just pushy salespeople trying to meet quota.