I know, lets just make things simple and use a viewscreen in the first place. Then you could put a few extra layers of hull between the people controlling the ship and that nasty vacuum stuff.
Why? No one has yet made a case for the crew being in anymore danger whether the viewscreen is backed by a wall or truly transparent.
How do we even know if the material used to make the viewscreen is in anyway weaker than the rest of the hull?
In point of fact, for a variety of reasons the current bridge placement is probably considerably safer than one buried deep inside the hull. This seems counter intuitive since you immediately think about the bridge being exposed to weapons fire... until you think back and remember that the kinds of weapons these starships are throwing at each other are powerful enough to glass whole continents in less time than it takes you to eat a cheeseburger, in which case it doesn't really matter how much material there is between you and the hull since anything that penetrates your shields is going to slice through your hull like a banana cream pie.
So the enemy hits you, say, just below one of your phaser banks with a disruptor blast, the blast penetrates your shields and carves a path of destruction thirty meters deep through the middle of your saucer. Lots of things explode, fires, plywood furniture painted gunmetal brown falls on people, etc. If your bridge is in the middle of the saucer section, then everyone there is probably dead or injured. If the bridge is on the TOP of the saucer section, then somebody's console exploded but they're still in the fight.
All that means is you don't get to roll a critical strike by accident: positioned on the top of the saucer the only way to destroy the bridge is to
actually hit it, and in nearly fifty years of Trek canon, no one has ever done that
on purpose.
Of course, Probert wanted to put the bridge on the front of the saucer rim and that is, in almost ANY case, the best possible place for it. But that ship has sailed, and two different starships have put a bar there instead... cest la vive.