The answer to this is not submitting to outdated forms of warfare, it's building more trucks. Many, many, many more trucks.
We have no idea what to do with all the oil anyway.
Cavalry as an breakthrough force is completely outdated eh. If we are relying on them to do that against the Wehrmacht, we would be beyond fucked.
Cavalry, by now, is ersatz motorized infantry anyway. They fight on horseback pretty much on only one occasion - when enemy is at the charge distance and is not aware of it.
And while proper motorized force would be nicer for exploitation - breakthrough is done with artillery and infantry, exploitation with everything else, not the other way around - we cannot be sure that we would have that properly organized and well-equipped motorized force.
And should we lack it, having an ersatz in form of cavalry is better than having nothing.
Reminder that the Old Guard were also responsible for getting a bunch of more competent military officers killed later on in the Purge.
Like, these are politically influential guys who're going to use that influence to wreck our military!
1) We would have more PI to save these guys then, if we go for expanded list.
2) This RKKA Old Guard are not the only ones flinging accusations like candy and it's Christmas out there. Taking out Budenny does not magically save Egorov, or Blyukher, or Tukhachevsky. It might help, but it is not certain.
Now, if we had an option to go for Yezhov, and install Beria early... And then, 2-3 years later, go for Beria and install, for example, Malenkov (He was Stalin's favorite for NKVD head at some point during the purges)...
That would have been ideal. Two consequent internal purges of NKVD would have meant they wouldn't have had time for anyone else