Oops. I meant for IS forces that seem to be moving away from Omnis. Clans still go hard on them.
No Inner Sphere army dedicated themselves to OmniMechs as much as any Clan has, and definitely for not as long.
Clans had Omnis for almost 2 centuries before they invaded the Inner Sphere, so got stuck in to certain habits.
Meanwhile the Houses have only known about Omnis for about 100 years as of the current timeline, and they often went with cheaper Battlemechs to provide numbers in the field. They also never went away from providing Vehicle rides for Infantry (PBI or BA) as part of regular doctrine, so as I said before, it won't be too big a change for the Houses, and one they will plan for.
Certain Mercenary units may have to update their tactics and methods, but I'm not exactly up on who are that reliant on either OmniMechs and Battle Armor to address that aspect.