I don't want to burst anyone's bubble about Dak because I like him, but there's always the possibility he could be traded/packaged for a higher pick later on. Despite what people might predict about his health, behind this OL I see no reason why Romo couldn't play until the end of his contract. If Dak resigns and is here in 2020 great, but if he continues to impress whenever he plays, a lot of teams will want his services and Jerry will always be Jerry. Also don't forget, Romo hasn't played in as many games as other starting QB's past 35. Only Palmer comes close right now, but our OL is much better than his and we have a better stable of RB's as well. For these reasons, I think Romo probably plays until the end of his contract.
Total games played including postseason
Favre - 326
Peyton - 293
Brady - 256
Palmer - 164
Romo - 155