A number of airports already effectively have some or all of this in operation, and there are serious disadvantages.
The main question is what do you do with the pax after they have checked-in, deposited their checked bags and cleared security? At airports like TXL, the only option is to sit in the sin bin for as long as is necessary, with only the gate lounge's own duty free shop for company (size: three strides x one stride). It is one of the most unpleasant waiting ideas that I have seen.
However, the alternative at TXL is to check-in and deposit baggage, and then go off to the lounge or some other landside place. But this does nothing to solve the problem because pax will be as late back to security as they are anyway.
At airports like SIN, where security is done at the gate, there is a similar sin bin problem, best solved only by turning up at the gate at the very last minute.
The idea of shopping for everyone is also great in theory, but pax are much less likely to be able to use the facilities because almost everyone is going to be wheeling around a trolley full of bags that will need to be checked. Going shopping when you've got that in tow is not very appealing. And shops (and the landside space generally) will have to be much bigger to even try to accommodate this.