For me the 175 purchase (at least the initial 35) could be justified purely to facilitate organic expansion, and to replace the Dash on some "prestige" routes:
- the example of the latter they have already trumpetted is LGW-GCI, which we are told will get the first 175. I expect there are others that fall into this category (operated from the main bases)
- the Q400 would continue to be utilised on many existing routes where there is no benefit in deploying a pure jet, with a few more seats (ie, many short UK domestic routes), and will be used to expand the route network and frequencies as demand picks up.
- the 175 could then be used on many routes, where we know the Dash can do the job (and does today, eg, SOU-VRN, the IOM-PMI charter), but where a jet would be preferable. Also to expand the network with new routes that also better suit a jet (because of distance and also because they should achieve the necessary yields) from bases like SOU - the publicity that came out around the 175 purchase suggested that this is one of the ways the new a/c would be used.