How secure is an airplane on the ground whilst passengers are boarding the aircraft?
Those guys would gain no more than they'd gain if they hijacked the bus taking you to the airport.

If fact they'd gain less as they'd have nowhere to go and there's a lot more (armed) police at the airport to take them down than there is in some remote place they could drive that bus to.

The greatest risk to airliners isn't people forcing their way into the cockpit on the ground who are among the passengers waiting to board.
It's in the ground crew who can (at least in theory) bring aboard almost anything they wish (including bombs) with very little oversight (in fact this has happened in the past).
And there security has been seriously tightened (and it needed to be) over the decades, making that scenario far harder.
I won't say impossible, because nothing is impossible as long as there is a human factor involved anywhere in the chain, but bloody hard.