We may be to the point where it's completely irresponsible, in both a logistical and fiduciary manner, for major airlines the size of WN/FR/etc to rely on a single aircraft type.
There, I said it.
At some point, the sheer inherent risk of having your entire operation bound to the welfare of a single manufacturer/airframe, could outweigh the obvious and well-known benefit of doing so.... and I'd argue that we're there.
As bad as this MAX debacle is, if could've been a whole lot worse simply with the addition of time. Instead of 2019, had this occurred just a few years down the road, these airlines would be devastated, as a far larger percentage of their fleet would've been grounded for a year, with no clear return in sight.
And for anyone who thinks such a scenario is a stretch: realize that it took 27yrs for the 737's rudder issue to manifest, something which in itself almost got the entire 737 fleet grounded as well.
So while these carriers may talk the talk, in terms of remaining "loyal" to their preferred airframe/manufacturer/etc; there's NO QUESTION that their boards have various departments recalculating the longterm outlook of cost vis-a-vis risk, in terms of fleet and supplier diversity, in light of this. No question there, whatsoever.
The only question is how we're going to see it manifested. FR's already giving us hints, let's see if it follows through.