Yes, Yamaha has been weak for years (last years of the Rossi area, and Rossi oft say this the engine is to weak), and the advantage of the in-line engine has become smaller and smaller because Ducati has also become stronger. I think if Ducati had given Lorenzo another year or two, he might have been the next Ducati world champion after Stoner.
In the golden era of Valentino Rossi or before Valentino Rossi was injured in 2010, the Yamaha motorcycle was also still in the weak category, but it could still be world champion in the hands of Valentino Rossi and in 2010 it could still be world champion in the hands of Jorge Lorenzo. This means that a Yamaha motorbike will be very extraordinary if there is a rider who knows how to ride it very well, because to understand a motorbike like Yamaha is not so difficult even though its character is different from other manufacturers' motorbikes.
Ducati usually changes a lot of the bike from one season to the next, so Ducati always needs some time to understand the bike, see last season and this season with Bagnaia, it always takes them some time to understand everything with the new bike, maybe they often make one step too much, and Yamaha one step too little.
Yes, Ducati changes their motorbikes and motorbikes very often every year, so when Jorge Lorenzo is at Ducati it is not immediately competitive because Jorge Lorenzo still needs to learn how the character of the Ducati motorbike itself is. And as for the world champion in the year after 2018 (which was Jorge Lorenzo's last year at Ducati), it will be very difficult to get a world champion because Marc Marquez is still very strong in 2019 and it is proven by the number of victories that Marc Marquez has achieved is very large. and even Marc Marquez almost always finished on the podium that year (both podiums 1, 2 & 3), but it was not managed by Marc Marquez because he himself fell while leading the race at his favorite circuit in Austin Texas.
Honda has completely lost its way and Suzuki is just a shame, you can see what you can do with the in-line engine, in-line engine is not an excuse...
Only manufacturer Honda has never won this season, so it's worth saying it's lost because Honda itself can't do much when the season is running. For Suzuki, I think it's a little different from Honda because the Suzuki factory has won the Australian Phillip Island race, but what's really unfortunate is that there will be no Suzuki factory again next year, which means this season is the last season for Suzuki.