Depends on what approach they took to replacing them. There isn't a UK gauge tilting 125mph DMU, no, which is probably why they've stuck around. So any viable replacement is going to mean changes to the service pattern.
I have wondered, though (and this is completely opposite to my "if it ain't broke" line upthread, and is heading into speculative ideas, so we may need to split the thread if it goes too far that way), if you could for instance sell moving the North Wales Coast services to the Chiltern line via Shrewsbury (thus slowing them down somewhat) but offset by new rolling stock with very high quality interiors (I'm not going to comment on what, as there are loads of options, some new and some used) and an hourly service to Holyhead rather than a bitty one?
So you'd basically be recreating something like WSMR for the North Wales Coast but in a franchised operator? Perhaps it could even be operated by TfW, then it would be controlled by Wales and run for their benefit?
Then you'd only need EMUs for the WCML operator, and as Alstom appear to have bidded a tilting HS EMU for HS2, getting some of those built sooner rather than later to run on the existing WCML (for now) could be a good plan. Then rework some Pendolino coaches to get them all to 11-car (yes, I know it requires addition of a transformer, but if you can turn an old London Underground unit into a viable branch line DMU...
) and you're sorted.
Edit: another similar option might be to get the wires to Chester and follow the same EMU based plan, but then to add an hourly InterCity quality (68+Mk4?) Holyhead to Birmingham timed to connect with it at Chester? Or even bin the wires off and Crewe? I think total loss of the through service would perhaps be politically too much, though.