I don't think The Witcher 3's main story was TOO LONG. But I gave up long before the end, not because the main story was too long, but because of the innumerable side quests. You really need to pick and choose which ones you will need to do. Moreover, doing even half the side quests had you overlevelling for the main quest, to the point where you might be level 20 and still have level 8 side quests that weren't done and the main quest progression had you looking to do level 13 quests. By that time, you aren't interested. Not because the quest isn't interesting per se, but because you're already WAY over-levelled for it, it will provide no challenge AND it will yet increase your progress to even higher levels.

Now I understand such a progression mechanic might be useful if you are playing on the hardest difficulties, but that's a problem, since some people are completionists on the first playthrough and don't want to set the game to the highest difficulty just yet.

So I don't think shortening the main quest is a good decision. I think not letting the side quests create a story spaghetti nightmare is the key. But that depends on how the game is structured to begin with. If they are keeping The Witcher 3's structure, then I really don't know how they can achieve this other than, sadly, shortening the main quest.