That is not relevant. I'm on the plan of my original service provider, regardless of whether or not that plan is offered at my current location.
You have access to a nice plan - and that's good - but not all EU countries have nice plans and cheap prices. Someone in, say, Portugal doesn't have a local "100GB for €20" offer. They probably also can't get the same offer you can because your provider will ask them for a local address, a local bank account, there will be a different language, etc.
Most providers also have rules on how long you can use roaming for, so it's unlikely that you can just pick a plan from a French provider and use it in Germany (or multiple countries) for a year straight... For that you'll need a travelling eSIM or something like that, but is anyone offering a travelling eSIM with 100GB for €20 a month?
Essentially your mistake is assuming that everyone in Europe can get 100GB for €20. Some can, some can't. And that's the problem.