Non-issue.
There were different SIM card sizes used over time, from standard to micro to nano. Every smartphone manufacturer long enough in business changed from a larger size to a smaller size at some point.
Most SIM cards nowadays are produced in a way that you have the standard size originally, but you can easily break the size you need out of it yourself without tools.
So you could easily change from standard to micro or standard to nano or from micro to nano if needed.
If you couldn’t, complain to your SIM provider, not Fairphone.
For SIM cards which don’t work that way or are nano already, there are adapters to get a smaller size into a larger size slot, and since the SIM chip layout is compatible and there are tools for it, you (or any phone shop) can cut out a smaller size out of a larger size (although providers don’t encourage or support that, they more like to send you a new card for a lot of money of course).
To specifically irritate you.
Because it was designed that way.
That’s a pity, because the Fairphone 1 will not come back … but you could make your points here …