I don't think allowing for infinite string length encourages interesting strategies. Right now you could choose lower difficulty tricks for two minutes and make all the other competitors cool down. Also, by using easier tricks to extend your runs it also discourages density and variety. I'm not saying that this was a particular problem in the variety around from this year's competition - I think all the competitors did fantastic. However, I did measure the number of contacts for each run and the string length significantly decreased over time:

I think with a finite time limit (i.e. 50 seconds) you may have players end up performing more consistently over the competition. I would rather see three 30-50 contact strings over one really long one and then two significantly shorter ones. Variety and density should also increase because the player will need to make each contact count. If the player is consistent enough to make it through the entire time allotment they can finish with a hand catch or a pass which I think is also more appealing from an audience stand point (as opposed to shredding till you drop). I think it would be amazing if a rule like this was implemented to see the bag go all the way around the circle without dropping effectively achieving a full hack in the highest level of competition.