Whilst it is true that any upgrade of the West Coast Mainline is likely to be expensive and disruptive, truthfully building a HS2^▸ (The next High Speed line(s)) line will disrupt peoples livelihoods much deeper than upgrading existing lines, also the countryside doesn't need trains racing through at 250 mph.

When you get to trains longer than five or six coaches which need to do more than 115mph though, I don't think there's much in it between MUs^▸ (Multiple Unit) and loco+coaches+DVT^‡(resolve) sets. With long formations, the high cost of the loco starts to be balanced out by the cheaper unpowered coaches. Also, with the higher speed the MU alternative is unable to have corridor connections on the ends and parts of the leading vehicle have to be locked out of passenger use for saftey reasons, meaning the non-passenger-carrying length of the loco is not really a problem. Not much in it then, and both options (LHCS^▸ (Locomotive Hauled Coaching Stock) and MUs) then have their advantages.

The Island Line is maintained by the TOC^▸ (Train Operating Company), there is no incentive for them to do any major renewals or upgrade the only requirement from the franchise to hand it back in the same condition.