Just think about it. Before they came into the game, there were barely any tier 8 only battles, in fact, that is why I never kept my t8 tanks. Why play t8 when t7 is just as much fun and I'm not paying more to get mowed down by t9-10 tanks?
Nowdays I get into t8 battles in average 5 times out of 10, the rest is either t7-6 or t9 and very rarely I get into t10's. But there is a catch. Often over 50% of the tanks in these t8 matches are filled with Defenders, Skorpions, Su-130s and alike. These tanks give the perfect reason why a t10 only player should play in t8. Their skills are most likely superior to the average t8 player not to mention their equipments and crews and those tanks are the perfect money makers to keep up with the costs of playing t10 tank.
This way WG can claim they "improved mm for t8 tanks" while doing nothing but make it worse (khm 3-5-7), then reroll it to its previous version AND claiming to improve economy for players in higher tiers (who are willing to cough out 60-120 dollars ofc)