I'd hazard a guess that the main reason for the TOC making this decision was in fact the leasing price difference. We know that given the respective dates that each fleet was procured, those price differences would be sizeable (as we've seen other TOCs make similar fleet replacements which seem unnecessary/uneconomical at first until one considers leasing costs). As whilst the fleet may well be an upgrade, I don't think it's a big enough of an upgrade to warrant the replacement (i.e. the 350s were still pretty new and weren't seeing passengers crying out to replace them as is the case with other older fleets that are around).
That being said, from a passenger point-of-view, I'm really glad that this economic quirk has meant that the 350/2s are indeed going sooner than we might have otherwise expected. As a regular traveller between London and Worcestershire/Birmingham I'm of course spoiled with four operators/routings to choose from (Avanti, LNR, Chiltern, GWR), but for peak-time travel LNR would usually always come out the cheapest by a sizeable margin; but there was something about the 350/2s which I really didn't like. The combination of the 3+2 seating, the lack of tables to do any laptop work, and the weird middle-of-the-carriage-door-segregated first class sections, all combine to give these trains a really small/claustrophobic feeling that is unique to them, which is had to fully explain. It didn't make for an comfortable environment that I'd want to spend anymore than a short journey on.
And this combined with the often endless station calls, long dwell times, and often loud and/or anti-social behaviour of passengers using these trains no matter what time of day it was, really give LNR's 350/2s services between Euston and Birmingham New Street a 'cheap/budget' and 'metro/urban rail' feel, meaning I'd really try and avoid them and either pay a bit more or travel at a different time where possible to 'upgrade' to either Chiltern or Avanti to avoid them, and I wonder how many other people making the same journey do the same.
The new 730s are definitely a noteworthy upgrade, and are much more suited to intercity travel than their predecessors, so if LNR can make a big enough splash of it they might be able to win back a chunk of the London-Birmingham 'business/professional but don't want to pay Avanti's expensive post-COVID fares' market from what I would guess mostly sits with Chiltern at the moment, but probably some from Avanti too.
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