In a perfect world, the Engineers would do what they do best: combat engineering. The skill sets are similar to pioneers, but they aren't the same, hence, two different trades doing two different jobs. To an outsider (eg: me), they looked the same, smelled the same, etc, but they were certainly different.
Having said all that, in order to man the support platoons of old, we would need to "invent" PYs in order to do so. Where do we find those PYs? Yes, we used to have them, but they are long gone. Getting back the capability is the first step. If required, and if spread evenly across all four companies of a force employment battalion (in theory), then of the 36 sections, you end up with 72 infantrymen who are qualified "Pioneer-Lite". A crafty CO could use them how he pleases, and yes, some of his Sr NCOs and WOs would have the "adv pioneer lite" to supervise, as required. But it would come at the expense of riflemen.