Whilst I agree that there are obvious capacity constraints and no-one wants trains terminating in the middle of nowhere/mass, confusing bus replacement, I'm surprised by the plan for this.
Yes, I understand Virgin's service revisions, with MKC>Stanmore and BHM>Marylebone via Chiltern as key routes, it makes sense.
However, I do not agree with LNR tipping everyone out at Hemel Hempstead. If Watford Junction is available, you can turn trains back in platform 9. If you run them as 8/12 car, you CAN get away with 3tph which is not ideal, but it will do and many would have taken the MKC/Birmingham options anyway. Hemel might have a big car park, but it is wholly reliant on replacement buses which will have to use the A41 or M1 - it's not particularly robust, plus there are no amenities there and the station is smaller than Watford Junction.
I also do not buy the argument that the bus station is full/ there is no room for rail replacement - after all, even today there are about 8bph in rail replacement buses (some acting as DC line replacements, some providing the only service to Apsley and Kings Langley and some doing Hemel>Watford>Stanmore). Any rail replacement buses needing to stand can drop off at the bus station, and go to stand at one of the several lay-bys hardly used (I'm thinking Exchange Road, ones outside Sainsbury's, at a push the one outside WHC).
From Watford, the Met line is often empty off-peak as far as Harrow, there is definitely capacity and you could definitely provide a shuttle bus between the tube station and Watford Junction - the bus station is not that congested at weekends, definitely not on Sundays as there are 6 bays and most routes bar the 142/258 (which stand on Woodford Road) and Harry Potter 311 are just 1bph. Regulars will know that the 142/258 link up with the tube.
At least people would have had options i.e.
Watford>shuttle>Met line (direct to Euston Square)
Watford>rail replacement>Stanmore
Watford> 142 or 258 or DC rail replacement> N London