You can lose games due to punters, kickers, long snappers, etc. Every position can screw up and cost u a game. That shouldn't be the barometer to use a top 15 pick on.
But guards and safeties don't move the needle for a franchise and change the fortunes of a team.
You can find good guards...good safeties...good LBers in FA. ProBowl level players
Go try and find a probowl OTackle in free agency whose in his prime.
A lot of teams will draft LBers, safeties, guards in any round, and if those players become great players they'll just let them walk in Free Agency because they don't want to give a huge contract to a position of low importance.
Not trying to pass this off as some novel idea: countless GMs openly talk about the premium positions I mentioned, vs lower tier ones.