Whe did San Francisco bring in Steve Young?
Why did Seattle draft Russell Wilson after just paying Matt Flynn huge money?
Why did Baltimore draft Lamar Jackson will how much they owe Flacco?
Why did Kansas City draft Mahomes when they were paying big money to Alex Smith?
Why did Washington draft RG3 and Kirk Cousins the same year?
Why did New England draft Jimmy Garrapolo when they had Tom Brady?
Why did Dallas draft Danny White when they had Roger, or bring In Beurlein when they had Aikman?
Because you should ALWAYS be looking for and developing quarterbacks.
You're not doing well here.
We'll take these in order. Flynn was signed to a 3 year deal for 20 mil or 6.3 mil a season and only 9 mil guaranteed. The shawks then drafted Wilson the same year and they competed for the starting job. Wilson won and Flynn lasted only that year and his contract wasn't prohibited to being cut. Neither were already the starting QB when they either got there - not even close to the same thing.
Lamar Jackson was drafted this season because the ravens weren't happy with Flacco's play in 2017. After drafting Jackson the ravens quietly tried shopping Flacco but got no takers. Again not even close to the situation here.
Mahomes was drafted in 2017 which was Smith's 13 season and that would be the time a team would start thinking about the future - again nowhere close to the situation in Dallas.
RG3 and Cousins were drafted the same year to compete for the starting job - that's not even remotely close to the situation here, it's not even close in another universe.
Jimmy Garrapolo was drafted in Brady's 18th season, and was drafted to be a backup and maybe someday the starter but then Brady made his statement that he was planning on playing 5 more years. Again nowhere close to the situation here.
Danny White was drafted in Roger's 8th season but you have to remember he started FOUR YEARS LATER than most QB's because of his naval commitment. If it was normal, that would have been Roger's 12 season and again that would be the time a team would be thinking of the future.. Nowhere close again.
Actually you are 0 - 6 on your examples of trying to justify bringing in another QB to compete against a QB that will be getting real starting money and no 1st round pick even if the Cowboys were foolish enough to even contemplate this for even a second.