Pritchard has played a total of 7 minutes of football above Championship level in his entire career. You're not seriously comparing him with Dembele on those terms, are you?
I get you're a fan of Pritchard (I didn't watch Brentford last season so I don't have an opinion one way or the other). But dismissing Dembele because he didn't start many games last season, while at the same time championing a guy who has never started a single one in his life (at the level we're talking about) is odd.
For the record though; Dembele played 1841 minutes last season. That's more than 20 full games across all competitions. It's also a fair bit more than Paulinho (1387 minutes / 15+ matches) or Capoue (1282 minutes / 14+ matches) or Stambouli (1440 minutes / 16 matches). We've lost those three from midfield already this summer (assuming the stories about Stambouli are true). Plus we also need to make up for Lennon's 587 minutes* (6+ matches) and the 3 matches that Sandro and Holtby played early in the season.
Just because none of those players finished the season in the first team doesn't mean the time they did spend on the pitch can be conveniently ignored. So between Paulinho, Capoue, Stambouli, Sandro, Lennon and Holtby, we're losing guys that played 56 full matches between them.
I am mystified by anyone who can confidently state that two youngsters (Pritchard and Alli) - both of whom are untried at premier league level (let alone "challenging for top four" level) - are absolutely ready to handle 56 matches in a season between them, at the quality we need them to be. On top of that, you seem positively eager to lose Dembele also; leaving Pritchard and Alli having to bear the weight of 76 full matches between them.
I'm all for giving the youth a chance. It's working for us. But what you're suggesting is just reckless. It'll only take one of either Alli or Pritchard to fail to handle the pressure (or for Mason's level to drop) and we can pretty much say goodbye to the entire season. It's one thing to give the youth a chance; it's another thing to send them out there with no experienced leadership and little backup... and demand they succeed under the brightest spotlight in world sport.
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* I'm including Lennon in this because there's a lot of folks seem to think Pritchard would spend some time at least on the right wing if he were to be part of the first team.