I agree with you, a win in any GT is better than a second or third at the TDF. However, a win at the TDF is worth at least 3 Giro , Vuelta or combination thereof wins in my opinion. Winning the TDF is the ultimate prize. I also know if you don't ride it, there is zero chance for you to win it.Is podium at the Tour worth more than a Vuelta win? Cause it's really not in my book.
I think back to 2014 when Nibali wins the TDF after Froome and Contador are both forced to abandon due to crashes. Nibali was lucky that he didn't ride the Giro that year as he probably wouldn't have won the TDF if he had (although he did win by over 7 minutes). Furthermore, Quintana won the Giro in 2014, but passed on the TDF. Could he have beaten Nibali that year in the TDF? I wonder if Nairo looks back and asks himself "what if?"
Therein lies the dilemma, do you go for glory and try for the TDF win? or do you go a "safer" route, which, of course, doesn't guarantee victory either. Again, my opinion, but I don't believe one can be considered to be a "great" GC rider without winning the TDF at least once.