You can’t just simply “cut” all of UA’s 757 routes. They were served profitably for years and now you think 90% should just be done away with? I think EWR-KEF/SNN/ARN/OPO/LIM could all be sustained with a different aircraft.
First off, whatever was "served profitably for years" has little bearing now. We had been climbing towards the crest of a demand wave, and we've very suddenly crashed down into a trough. Load factors above 20% are looking great right now.
I take no joy in saying this because EWR is a really cool airport with lots of interesting international routes added recently, but you need to adjust your expectations regarding what is coming. EWR this summer is not going to offer remotely close to the number of flights and destinations that it did last summer - it would be very lucky to get back to 2019 levels in Summer 2021, but even that might be very wishful thinking. It sucks, but it's the reality we're facing. Some international airlines will likely pull out entirely (a la VS). UA will cut routes. For starters, you can probably expect any new route/frequency that UA had announced for this summer, and maybe some of the ones started in Summer 2018, to be cut, as they are low hanging fruit. I'd be very surprised if UA metal visits KEF, OPO, NCE, NAP, PMO, or PRG in 2020.
UA retiring the 752s seems likely, given that they have stated the 757s are first on the chopping block, and given that the chances of UA not
retiring any fleet types are approximately 0%. If UA does
decide to retire the 752s, they're certainly not going to upgauge anything unless capacity to that destination is drawn down elsewhere (e.g. my example where ORD-EDI and IAD-EDI could be cut while EWR-EDI could be upgauged - a best case scenario). If 767s are also retired, that makes an upgauge even less likely. Any route that is unique to EWR and is served by a 752 is vulnerable to being cut, plain and simple. UA won't have the right aircraft to fill those gaps back in until the 321XLR arrives. At that point, I certainly think long/thin transatlantic and South America routes are back on the table, as stated at the end of my original post. I don't expect UA will fly the MAXs on transatlantic routes, as they aren't configured for international service.
If you still have doubts about UA cutting routes, look at what AA just announced for PHL, their primary transatlantic gateway (through July, August, and September, AA's only TATL routes from PHL will be LHR, MAD, and ZRH). I don't expect UA will cut EWR back to that level as there is much more O&D traffic, but to be honest I think the only truly safe long haul destinations through this summer are GRU, DUB, LHR, CDG, BRU, AMS, FRA, MUC, ZRH, TLV, and NRT or HND. Anything beyond that will be gravy.
This is my signature until I think of a better one.