The NFL isn't made in the salary cap era for success only through drafting. At most, you get three starters a year (generally two, but we'll be generous) and they may not be quality starters. Right now, we have four from 2020 (Lamb, Diggs, Gallimore, Biadasz), none from 2019 (although Donovan Wilson and Pollard play key roles), two from 2018 (Vander Esch and Gallup, although Armstrong might be considered a starter now), none from 2017 (Lewis and Noah Brown are contributors) and three from 2016 (Elliott, Prescott and Anthony Brown for now).
That's nine starters over five years, push it to 11 if you want to count Wilson and Pollard as starters, so it would take 10 years to completely replenish your starting lineup through the draft, not to mention adding quality depth. Meanwhile, you are not holding on to most of those starters that long, so it actually takes even more time unless you really get lucky with undrafted free agents or low-cost signings.
You don't have to dip into FA because you whiffed in the draft. Dallas' draft success percentage is about as high as any team's. You have to dip into FA because you just can't successfully fill all your needs in the draft. When you could hold on to all your players as long as you wanted, it was possible, but salary cap-era free agency makes it impossible.
I haven't looked it up, but I would bet that there's not a team in the last 10 years that won a Super Bowl without signing at least one high-cost free agent.