I bought a ticket type that allows me to a cabin bag of a particular size. The finnair app tells me that for this specific flight, I can take a bag of that size.
Why would I have to pay to check a bag (or even worse — buy another, smaller bag, specifically for flying on finnair) when I’m entitled to bring the bag I have?

The onus shouldn’t be on the customer to check the plane type etc. (especially on connecting itineraries). If there were restrictions, they should be communicated (at booking or in the app)