Yes dude, it's ABSOLUTELY about that they are cheap, and available, and replaceable. And, they do sound good...I've always said that...but they don't sound as good as the stuff those players keep at home, for the studio.
This really isn't debatable.
Gary Holt, for instance...tours with his Jubilee reissues. They sound great. And, they are easy to find AND can be replaced IF someone steals his gear. At home, he has his Langner modded Marshalls which are irreplaceable. If someone stole them, he'd be heartbroken because Todd has passed on and good luck finding any other.
EVH never toured with his magic Marshall in the early yrs after a bunch of his tour Marshalls were stolen. Lesson learned, and thankfully he didn't have the 68 on the road when it happened.
You go ahead and believe your little 'budget amp' theory all you want. But anyone else with any common sense, and has played out knows that at any given time someone could try and steal your shit.
Playing with amps/gear that sound good enough, gives the pro peace of mind knowing that IF they lose some gear, they can head to a local GC and grab another DSL or 5153.