1910 wasn't a particularly good Republican year, and the argument can be made that the tides were flowing in a Democratic direction by 1912, even aside from the split in the Republican party. In other words, Taft could still possibly lose in 1912 without Roosevelt's involvement. Of course, he'd have a much better chance than he historically did. I am not sure that him keeping us out of war would be enough for him to be remembered as a Great President. Sadly, generally speaking, Presidents are better remembered for how they dealt with catastrophic events rather than the extent to which they avoided them. If American was not involved in WWI, you can bet that there would be grumblings about the impact of that in some American political circles. After all, they won't and cannot know what the result of our involvement could have been, but they will be well aware of what the impact of our neutrality was in their history.
Besides, while Taft will certainly set the stage for continued American neutrality, if the war lasts through 1917 isn't he going to have to share the credit for that accomplishment with whoever wins in 1916, which probably will not be Taft.