I said the same thing about Moncrief last year. If you get a reasonably high draft projection, you should go these days.
Now that the NFL has protected themselves from the insanely absurd money that was getting thrown to first round picks, there isn't nearly as much motivation for a player to stay. Moving up from the 3rd to the 1st round isn't a huge deal.
The bigger deal is getting your clock started, so that you can get to that second contract, where the big money comes into play. Plus, especially with football, you only have so many hits in your body, and it's best to maximize the number of them that you take while making money.
I'll never advocate for a player to stay anymore unless there is question about whether they'll be a high round draft pick (1st three rounds) or unless they are dealing with some sort of injury that held them back from showing their full skill set their junior year, and that injury is keeping their draft stock significantly down.
If Dak is a Top 3 round projection, he'd be smart to go. The extra year is usually not worth it, even if it's a strong class at your position.
Moncrief was in a ridiculously deep WR class last year, and he'd probably be a 1st or 2nd round pick in this year's draft, but he still made the right move to jump at the 3rd round last year. I'll say the same for our early entries next year. Tunsil, Nkemdiche, Conner, and Treadwell would all be dumb to stick around for a senior year unless they have injuries that significantly hamper their stock.