the MML and HS2 are separate projects, so it'd be great if we could keep them on separate threads.
The thing is, both of what is being said on this thread is true. There will be a capacity shortage on the southern MML post-Thameslink, and the stations at the northern end of the MML could do with a better service, as there probably is a problem of suppressed demand, given the rail service is not the best quality, and the M1 provides an easy route to the home counties & London. Not increasing capacity on the MML will lead to a squeeze on the ability to serve stations to Leicester & beyond.
HS2 frees up capacity by allowing long-distance services to run with more stops, meaning the need for semi fasts, neither expresses nor Thameslink local services, is removed. This means you can give a greater service to intermediate stations (as the problem of conflicting services is reduced) as well as a more extensive service. So when Sheffield's getting 4tph to London on HS2, you can let Sheffield services call at St Albans, Luton Airport, Luton, Bedford, Kettering, Wellingborough etc. This will lead to MORE people using the railways, because the biggest deterrent to people using lines is having to change trains, whereas driving is direct. If a station has more direct services, competing against an ever more clogged-up motorway system, then the train becomes an ever more attractive proposition for most travellers.
HS2 meanwhile will be taking people off coaches and the motorways, as well as aeroplanes, by giving a much better journey time to the country's major destinations. It's probably helpful to point out that HS2 directly reduces journey times and indirectly increases capacity. Trains aren't 'taken off' the MML, they're slowed down to serve more places. But this gives a much better frequency to those stations, and, given the journey time including stations for the present indirect services, a shorter journey time overall.
For instance, a St Albans-Sheffield journey with a slowed MML would take 2hr10 according to current timetables, assuming calls at Luton Airport Parkway, Luton, Bedford, Wellingborough, Kettering, Market Harborough, Leicester, Derby & Chesterfield. But that 'slow' service is 25 mins faster than the current service, which, having two changes, is decidedly unattractive to passengers. Let's even suppose it also called at EMP, Loughborough and Belper. Still, 15 minutes quicker to Sheffield than currently. Even 'slowed down' services post-HS2 will usually be quicker than the current journey opportunities.
However, this isn't to say that other projects wouldn't also be good ideas. Personally, I'd support the reopening of both Quainton Road-Rugby-Leicester and Northampton-Market Harborough. This would allow services from Leicester into both Euston & Marylebone, as well as St Pancras, and give many towns good connections to the North on MML services.