Just speaking for myself, a long flight on an E175 is just fine with me.
So, here's why I think the LGA/DCA will not end well. I will preface this by saying, I assume they are precluded from using the AA code at DAL for at least three reasons. If that is not the case then throw this all out as that changes things dramatically.
#1 First, VX did not do well. They had two advantages. They had (at least according to the award sites) the best inflight product in the market. They also were very cheap. They didn't want to be that cheap, but even if AS pursues a price strategy as well the costs of an E175 per seat are MUCH higher than VX's costs were, plus AS costs are much higher than VX on top of the airplane related costs. So, they either lose more money or raise the price considerably.
#2 Raising the price (which is the primary goal of going to E175s as they try to fix the route profitability) will drive away the existing customer base, particularly when coupled with...
#3 Moving from what was pretty routinely considered the best inflight product in the market, to what will be certainly a worse than average product with the E75s. I see there are some CR9s in these markets at DFW, so it will not be the worst product, but under any scenario it is definitely a large drop in product quality and that also will lose a lot of passengers.
#4 Love it or hate it, the Virgin brand was worth something, and it's basically gone.
#5 Having said all of that, it's basically a fresh start and a clean slate for AS in DAL. They will bring a percentage of the VX passengers over, but a lot will be lost. They won't have the product breadth frequent flyer lock-in like they will have in SFO/LAX because they are so small in DAL/DFW compared to AA/WN. LAX and moreso SFO are completely different situations where they have viable breadth of service.
#6 The Alaska brand is weaker statistically than the Virgin brand in DFW/DAL. What statistics you ask? If you look at the portion of tickets sold under the VX brand with DAL origin in DB1B, it's 38% of seats, and about half of passengers. That means the brand is well enough established so that they are not dependent on selling in LAX/SFO/LGA/DCA, etc to prop up weakness in DAL. Sure, they had to use price to get there, but still those are healthy statistics for VX going against AA and WN in their hub. The numbers are much worse for Alaska. It's only 27% of DFW originating tickets are sold on the AS brand, and only about a third of passengers. The reasons are that they have been leaning on SEA/PDX/etc markets to sell tickets, coupled with relying on the AA code share in DFW actually hobbled the development of their own brand in DFW.
Bottom line, if they truly stick with DAL-LGA/DCA it will be expensive and take a long time to make profitable. I guess the question is what were the other options, and it is possible this was still the best one given that they were probably loathe to help DL in DAL or anywhere else? Doing MSP/STL/MCI/DTW/MKE-DCA/LGA would have been no day at the beach either. So, I'm not saying it wasn't the best decision available, but I don't think it will be a profitable one for at least a very long time.