If under a certain number of pax, exit rows do not need to be filled in line with CASA requirements.
Only ABP’s may be seated in the exit row and must be briefed so you can’t just have anyone moving in there without a briefing (CASA requirement).
On top of that, these are sold as extra revenue seats (economy x). Letting just anyone sit there would be akin to letting anyone into J class just because the seat/row was free.
Just to be clear, if no one has purchased exit row seats and number of pax now requires exit row to be filled, they will be filled by pax. Ground crew will in this case issue seat changes to meet the requirements.