If you have the means to get 1500 hours without working as CFI, pipeline, etc then you don't need a commercial SE, in fact in that circumstance it would make sense to do everything in AMEL to get the ME hours you need for ATP.
This would apply if somebody already has a bunch of PPL time, or if they own an ASEL and intend to build time flying their own plane.
In another era, lack of professional flying experience would have hurt you for regionals, but today all you need is 1475 and ATP requirements, doesn't matter to them how you get it. Of course training will be harder for a private pilot as opposed to a CFI or 135 vet.