The man with only one gun may also not be all that much into shooting; may have inherited it but never used it; may not be able to afford more than one (which suggests, but does not necessarily mean, lack of funds for practice ammo or training).
This statement is over-used and under-thought. It may have been true a century ago; today it is a relic of a bygone time. I know a number of people with one gun and none of them shoot regularly (or particularly well). Everyone I know who shoots half-decently owns multiple guns and devotes a fair amount of time and energy to maintaining that skill.
That said, I believe that there is something to be said for a single platform--especially if you do not shoot frequently. If you are comfortable with the Glock action and manual of arms, and you practice enough for it to be second nature, I believe you will be well served.
One of my many personality flaws is that I try to keep only those handguns that I carry or shoot regularly.
This gives rise to a number of conflicts. I shoot a 1911 best, and try to carry a 1911 exclusively, but I also own some S&W revolvers (K and N-frames) and therefore "must" carry them at least a certain percentage of the time...generally, winter time.
I try to spend some time at the range every week with my carry pistol. When the seasons change, I make sure and spend some extra time to make sure the "transition" is ia smooth one...yet I doubt that muscle memory is truly and immediately ingrained during the changeover period. Only in a true emergency is the theory proven, and if luck is with me, the true emergency will never confront me. Of course, in the absence of proof, there is always doubt...
If I were able to carry each for six months, I'm fairly sure I could achieve a balance...but due to our seasons, I have to work hard to carry a revolver four months of twelve.
I am a bit less concerned about the 1911-to-S&W revolver transition (after all, a revolver is as intuitive as it gets) than I am the inverse.
I am not yet ready to sell my revolvers, so I will deal with the cross-training, and the doubt...but I am not saying that this is the wisest course of action.
Perhaps these words will one day adorn my headstone.
My point...your choice. Make sure you can live with it.