The problem with saying "im playing because of hl" is you're using the real world 6s community in your hypothetical argument. To consider the growrh we lost or gained because of UGC, we need to consider if having only had ESEA wouldve made tf2 a more popular and reputable esport. If tf2 didnt take off because a ton of ppl werent playing 6s and sponsors didnt like the split scene, you cant say that HL was a net positive since the end result is tf2 being a smaller game and therefore not attracting as many players. Theres also the question of how many ppl wouldve just gone into 6s if hl wasnt around.
Too many variables and too complex a question imo. Youre discussing the fallout of a decision thats shaped the entire history of tf2, itd be difficult to say what wouldve happened in another reality