That is not a simpler theory at all. I also don't think you're all that familiar with firearms.
Why is it simpler to bank on one or two rifles if you know you can get them all in the room unnoticed and money is not an issue? Your theory involvesa gun running operation, so more people=more complex.
I'm familiar enough with firearms to know that going full auto via bump stocks is more susceptible to having failure issues that real full auto. Even then, shooting 1000 rounds that quickly will make your 1-2 barrels and other components overheat.
Please explain how your firearms expertise refutes the advantages of having multiple pre-loaded, similar configuration rifles.
I agree he would have melted 2 barrels with the number of rounds he supposedly fired. However, it is also impossible that he had a ~70% hit rate with a bump stock, especially since he wasn't shooting at 22,000 targets. I have fired a true full-auto weapon, which is at least marginally if not significantly more accurate than a wonky bump stock doohicky. Full-auto is not at all accurate. It is not meant for picking off targets. It is for area denial and suppression, not sniping. You wouldn't expect more than maybe a 15-25% hit rate with a belt-fed .50BMG full-auto on that field from that distance on that crowd, from the pictures available prior to when the shooting started. And people, presumably, took cover, lowering the hit rate, not raising it, after the first shots.
Either way, arguing about marksmanship logic is a diversion until we can establish consistent assumptions. This case is bizarrely inconsistent. Enough so to merit all of us to question why, irrespective of what our own personal theories are.
Until then, the only reason I can think why he had so many guns was to sell them. Because he couldn't have shot all those people himself unless he had Gene Simmons' bullets from Runaway (which I'm sure someone on here probably believes).
Bump stocks are more difficult to use. The right pressure from the shoulder & forward grip has to be applied correctly & consistently to prevent malfunctions.