Add to that the planetary garrisons. Using abstracted, Minor industrials have twice as powerful of garrisons as Other worlds, Major worlds are twice as strong as Minor, & Hypers are twice as strong as Majors.
If using the Detailed Garrison option, the more industrialized the planet, the higher the modifier. The more industrialized the world, the more infantry & armor kits it has, & the better the chance of 1 or more 'Mech regiments as part of the garrison. IO, p. 368
The main issue, though, is that ISAW rightly recognizes that it's much easier to destroy (engage in combat) than it is to create (build factories). Of the factions listed in IO, the Outworlds Alliance is the only faction that generates enough RP to have 100% of its forces engage in combat -- only needs to spend 38% in combat ops, so it has plenty left over. Everyone else is limited on how many units they can send into combat in a turn, but even at worst they only need to spend maybe 3 turns banking RPs before an all-out offensive. Even with rebuilding units in between, the 26 turns it takes to recoup the cost of the Minor World upgrade would allow for 1-2 massive attacks, or nearly-constant harassment a (attacking 1 turn with 10-20% of your forces, then taking a turn off to rest & preposition the next set of units). Sounds a bit like the Succession Wars era in canon.
Now, me, I like to mix both attacks & building in my Axis & Allies-type games. Spend a few turns pushing into another realm, then rest & build up a few more factories in my rear areas so that I can have even more RPs coming in, rinse & repeat...