Ranked at Tier 10 has noticeably gotten stale for a lot of people as the meta has amalgamated into a hulldown slugfest with only a few tanks such as the Kranvagn, STB-1 and Super Conqueror seeing consistent play. I do think the 10 player season contributed to this but tier 9 overall is better balanced with a greater diversity between strong tanks, for example the E75 and AMX M4 51 lead into non-competitive Tier 10's but are crackers at Tier 9. Tank destroyers at Tier 10 really aren't the best outside of handful of picks but at Tier 9 there are some really influential vehicles.
The biggest concern at Tier 9 is ironically the same one at Tier 10; rewards vehicles like the Concept 1B or Kamfpanzer 50T are objectively stronger than their tech tree counterparts and would dominate the meta. However, those really are the only two major contributors to the problem, with really only the T-55A also falling into this category. WG could easily exclude those 2 vehicles and possibly the T-55A while allowing the other reward tanks like the AE Phase I, K-91 PT, Obj 777 II and Char Futur 4.
Overall, I think Tier 9 would be radically better for ranked and would be more inclusive to players who don't have Tier 10's or can't afford to play ranked but want to. This would expand the playerbase and allow players to try different vehicles with different specializations from field mods to compete.