So is Witten. Old folk getting good money and delaying progress of the young talent.
Negative way to look at it. Unless your a top 50 pick your probably not coming in day 1 or possibly even year 2 and clearly outplaying both Witten or Matthews (even a washed up high level player is still an average NFL starter, most players don't even make an active roster for more than 2 years let alone be an average starter).
Veteran starters on 1 or 2 year deals allows the younger players to develop in the film/weight room and slowly rotate into the system as the season progresses (in Wittens case they'll probably be significant rotation even week 1). This is a huge benefit and it doesn't put any long term cap implications on the team.