Just because the junior are spread across all fleets doesn't mean the company can't furlough. If they really want to do that (hopefully not), all it takes is some deviation from past practices. Namely, monthly bids. They can cue up replacements for who they show the door with about 2-months lead time. Put them through ITU/IOE and have them ready when they pull the trigger on the first say, 50. Then just keep the same pattern going until they're done with the furlough. I saw 100 per month for almost 2 years at a past legacy. Not difficult and that airline had over 10K pilots.
With most fleets buying up lines and the bottom 100 pilots representing a max of under 20 pilots in any specific seat, we're not talking major disruption. With cross-base options in every aircraft except M57 (unless E57 is an option), there are also options to fill trips via deadhead if a particular base gets thin on pilots.
Bottom line is, if the company really wants to make the F-word happen, it doesn't take that much effort. Just a little pre-planning. So, I wouldn't hang my hat on the spread of junior pilots being a show-stopper. Whether management really puts any value on being able to say they've never furloughed is up for debate. They used to care about delivering freight on time, now it seems - not so much. Time will tell.