I always thought that this was to do with yeast mutating in an environment rather than infection. It's a bit of both, but for the pros lager yeasts are recommended to be repitched fewer times than ale yeasts due to genetic drift.
And I thought THAT was because of the adjuncts they use which makes the yeasties lazy and just go after the easy sugars.....If you use the "real" grains they have no choice..... or so I was told.
I've heard this before and I think it's BS. Yes, if you grow it exclusively with glucose then there is no selective pressure to maintain the maltase genes. But neither is there selective pressure to get rid of those genes, so the individual cells don't lose them instantly and their is not massive and simultaneous loss of the ability to ferment maltose in the population. They might stop making the enzymes, but that is not the same as losing the ability
to make them. Oh, and even your healthy population of DME grown yeast will go after the easy sugar first, the glucose.
I grow stuff on glucose all of the time. I grow it in glucose before adding glycerol and freezing at -80C. I pull it from there and streak it to a 2% glucose plate. I pick a colony and grow it in 3 mls of 5-10% glucose YPD, then 12-15 mls of the same, then 50-100 mls of the same. From there it goes into DME, and any cells that may have lost the ability to use maltose will not thrive.
I would make sure to use something with maltose as the last step before pitching the yeast so that the cells have plenty of enzymes for fermentation. But even if you don't, once the cells detect the maltose in the solution they will get to work on making sure they can use it. There may be a bit of lag while they build up the concentration of maltase enzymes, this might be the laziness that people mean.