I suspect it’s penny wise, pound foolish.
We’ll never know, but there’ll be downgrades, sure, but there will also be extra take up.
I’d frame it as just software features, so no interest, no free cash deposits, no card. You could even think about limiting the number of connected accounts etc.
The other consideration is increasing interest rates. Monzo will probably be in rude health shortly due to the margin they make on their deposits with the central bank.
That means they’ll be incentivised to increase their deposits, which could mean increased interest rates or (more likely imo) a lower tier which guarantees revenues but also brings in extra deposits because it cements Monzo as a primary account. It may also (hopefully) mean some Plus features making their way to the free account (limited free connected credit cards, anyone?)