Going through the wayback machine to see if leaving Texas actually worked out well for certain people...
The only guys I can recall the past 10 years or so who actually made playing time waves at Power-5 schools after leaving us in this fashion were Cayleb Jones and Darius James. Derick Roberson was epic at Sam State, and probably had the best career numbers of someone who transfered, but that's FCS.
2010: Darius White had 1 good year at Mizzou. I think Darius Terrell switched to H-back at UNT and made all conference for CUSA.
2012: Adrian Colbert got enough PT in Miami to turn it into a draft pick and a decently-long NFL career. Dalton Santos had an excellent year at La Tech and I think went immediately into grad school to coach.
2014: John Bonney was a GT for Tech and immediately started for them.
2015: Kai Locksley was a part-time starter at UTEP when he wasn't hurt. Ryan Newsome played a lot at Arizona State but never made a huge impact.
2016: Jordan Elliott had a really good career at Mizzou, and I was sad to see him go. It's weird calling Buechele a transfer, but he sort of is. Good career at SMU. Kyle Porter had some decent years at UH.
2017: Pouncey ended up at UF after badmouthing them to recruits, mostly because of his brother, but I don't think he played much there.
2018: Cam Rising is the only notable DI transfer, but hasn't done anything at Utah.
2019: Kenyatta Watson looked good in our limited time with him, but is now at GT and wanted to be closer to home. It looks like Javonne Shepherd might end up at A&M.
2020: JQJ is the only notable one, but the jury's out on when he'll be able to compete.
So, over the past decade, we've had maybe 15-20 guys who still got decent playing time elsewhere, compared to the 50+ who left the program and didn't do jack ****.
There are obviously a lot of factors in the works other than playing time. It could be grades, not feeling that UT is the right fit, girlfriends, parental pressure, or whatever. But I can't imagine any 4-or-5-star recruit thinking that transferring is his way to bigger and better things, looking at the track records of everyone else on this list. Unless you're Kyler Murray or Jalen Hurts, there's no doubt in my mind that guys like Jalen Green just aren't going to amount to much elsewhere if they couldn't hack it at Texas.