What are the financial implications for Lufthansa (and other airlines) of keeping the back row of their A320neo free of passengers?
Not being able to sell seats on the last row would (in Lufthansa's case) theoretically mean a 3.3% loss of revenue: LH setup for A320neo is 180 seats, 6 seats in last row.

But, only theoretically, because seat occupancy rate for Lufthansa Group was  81.4% (2018). It is very unlikely that all of the flights operated with A320neo would fly 100% fully booked. In a non fully booked flight, the last row would obviously be the first one to be left empty. The actual loss of revenue would be way less than 3.3%, but how much, it is impossible to say.

Anecdotally I'd say that it is a blessing the last row is out of service for now, this Business Traveller article will give you an idea  why: no window, seats do not recline, legroom is smaller...

World Airline News: Lufthansa Group becomes the largest airline group in Europe with 142 million passengers in 2018