Yeah, the price of magazines pretty much kill skirmishing with GBBRs. That, along with weather affecting performance, is pretty much why AEGs still dominate the field. Cold weather affects AEGs too but not as severely as it does GBBs. There are air-rig setups out there but those can be expensive.
The good thing that comes of this is that a GBBR got past the ATF. Hopefully, more will follow so that the King Arms M4 GBBR isn't the only easily obtainable GBBR to US customers. Not that there's anything wrong with the King Arms M4 GBBR, but I do know it uses aluminum internals while other GBBRs on the market have already adopted steel internals.