Just to add to this, every plane is vulnerable while on the ground. It doesn't really matter how stealth an aircraft is if your bombarding the hanger they're in with artillery.
But Russia doesn't even really bother with hangers. This plane was just out in the open, on the fucking tarmac, for God, observers, satellites, and anyone else who cared to see. Even stretching a fucking tent over the aircraft would have at least introduced a certain amount of guesswork to the strike. But no, Russia put "the pride of their airforce" out there for everyone to see and paid the price for it.