Yes, you're right that leveraged ETFs have done well over the past 20 years. Other people have already busted the myth that you can't hold them long term. There's a nice write-up here:
http://www.ddnum.com/articles/leveragedETFs.php It shows that since 1885, the optimal leverage has been 2x (optimal leverage since 1950 has been 3x).
However:
1. If TQQQ had existed in 1999, it probably would have gone down very close to 0 during the dot com bubble.
2. Google "XIV" if you're not familiar with it already, it's a nice example of a leveraged ETF that blew up. To be fair, it was IMO extremely obvious that it would blow up.
I think some of the leveraged oil ETFs blew up last year too. Is it possible for the S&P to drop 33% in one day nowadays to trigger a blow up in UPRO? Are you willing to bet your entire brokerage account?