Yes, it does sound quite ridiculous, but hear me out...
Lewis is breaking all-time records left, right and center and will without a shred of doubt go down in history as one of the greatest drivers of all time. But still, some will argue that his stats are inflated, not only by the sheer number of races (and bullet-proof reliability of his era), but also by the fact that he has won his titles against relatively weak team-mates in the best car by far.
The point here isn't whether or not that's true, but that this is a possible narrative that can be strung around his career.
Let's just say hypothetically that Alonso had been in the other Merc from 2014 onwards, and that Lewis would prevail in 4 WDC's against Alonso's 3 in this completely made up scenario. I'd imagine that this would be seen as a Senna/Prost-esque classic rivalry that would elevate not only Alonsos status, but also that of Lewis, even though (statistically) he would have a much weaker CV to show for it than what he has today. One could just look at Senna and Prost for instance, who both have weaker CV's statistically than Vettel, but whose legacy surely will be greater than Sebs.
In hindsight, the fact that Lewis lost the 2016 title to Rosberg may not have been such a bad thing for his own reputation after all. The paradox is that by losing the title, all of his successful title campaigns against Rosberg were suddenly won against "World Champion Rosberg" instead of against "Good-but-not-great Rosberg". One could therefore argue that Lewis' legacy benefits more from having lost 2016 than it would have had he won it.
So... am I on to anything here or just blabbering a bunch of nonsense?