You can up the quality of your team and win a challenging league. The problem starts the next season where the min. quality requirement is likely to rise (sometimes, very significantly, as in the case of the cup). For instance, say you need an upper 5 star team this season to properly challenge for the league, the next season, the min. requirement might be lower 6 stars, and it might keep on increasing like this after each season up to say a min. of 7 stars, where it will then generally stabilise except in rare cases where there is a 9 star team. That is only for the league. The Cup may become even more difficult as you will likely face teams as high as 3-5 levels higher than yours. This is why many users try to fool the draws by decreasing the quality of their team at the end of each season. That is why I say, if you play the game like you are expected to play, the Cup is the most difficult competition to win.
You may still win the Treble the way you are doing; it may come after staying as a much lower quality team than the others for a few seasons. But, it will be the lower batches' (2-4 stars) treble, and hence, significantly less prize money for the level concerned. For comparison, it can be said to be a second or third division treble. At least, you can win something.