One final thought: I wonder if masts keep "moving" back, with high aspect mainsail, shorter booms and self-tacking foresails, maybe travelers will become history, just like backstays in some cases!
I’m not quite sure about that.
Apart from sail shape (halyard/Cunningham, outhaul, mast bend), mainsail trimming is essentially about controlling the leech (open or closed, more or less twist). A correct leech tension will give power to the sail while maintaining laminar airflow and creating upwash for the foresail.
That’s why the main has tell tales on the leech, while foresails have it on the luff.
The higher the aspect ratio, the longer the leech for a given sail area, the more important a powerful boom vang will be (you’re right: downhaul is not the right term, I already made this mistake before but English is only my third language so please excuse me once again). Very low aspect mains such as gaff sails don’t even need a vang because the length (weight and lever) of the boom is mostly sufficient to tension the sail.
But since a boom vang is utterly inefficient in creating downward force on a modern rig, I consider a traveler for the main sheet essential for performance cruising, especially upwind. The further aft (and the longer!), the better.
By the way, when sailing upwind in good conditions, you want the boom amidships. This is impossible to achieve without a traveler pulling the sheet on the high side.
On our boat, until a beam reach, the sheet only serves as a (very powerful) vang. Sail power is regulated by the traveler which runs along the whole available beam in the aft cockpit, out of way for easy circulation. With a 6:1 purchase and within immediate reach of the helmsman, such a traveler is extremely efficient and easy to handle.
Further downwind a dedicated line is just taken down from the boom to the toerail and comes back to one of the coach roof winches. This is once again very much more efficient than even the most powerful traditional boom vang that has to work under an almost impossible angle.
Considering self-tacking foresails: the more the mast comes aft, the lower their aspect ratio. Without any ability to change the sheeting angle, these sails become highly inefficient already from a close reach because of excessive twist and spoiling in the upper part of the sail.
So I am indeed quite shocked to see that a yard such as X-Yachts, with even their Xc cruising range designed with good performance in mind, give away an essential feature such as a main traveler.