It was really inconvenient to switch to an MVNO or another carrier. You almost always had to order a SIM and wait a week for it to be shipped. Right now, I can activate an eSIM and be using the plan within like 10 minutes. I also am able to continue using my current plan.

In my current setup, I have a plan with primary carrier for my primary number, then a backup data only eSIM with non-expiring data for if I'm SIM swapped or overuse my data plan on my primary plan. I've actually tried and used different MVNOs.

It can be made difficult if you want to keep your number. Years ago I worked for a startup mobile operator. The incumbents didn't like the new threat, and did all sorts of shenanigans to stop growth, things like paying people to object to new cell sites etc. One of these things was releasing ported numbers on the last possible day. So, while you had your new device and SIM card, you didn't yet have your old number. The old operators legally had 30 days to release/transfer the old number, and would wait for 29 days before doing so. You'd be surprised at the amount of contract cancellations this would cause, with the customer thinking it was the new operator being useless. The legislation ended up being changed to a more reasonable timeframe because of this anti-competitive behaviour.