No, I think it's mainly because the new system of rentals make it easy for an account to reach Diamond or Champion with a single day of rentals. And then they don't have to spend anything else the rest of the season. Players like @jacekw have already demonstrated how this can be done.

A botnet or a human player can just spend 1 day farming each of 15 different accounts, renting the same set of cards each day (for each account) and get 15 season end rewards.

With this upcoming change, in addition to 1 day of playing, it just means people will also need to maintain their rating/league for the final day of the season. More importantly, it also means that a given card can only count its power towards one account on the final day of the season.