Getting people through PTS is not actually too difficult, but there is cost and time associated with it and it is an example of the sort of restrictions faced on the operational railway that aren't there on a new-build site.
More critical is the fact that you can't just crack on with the job like you can on a 'green field' site, where you can work 8 or 12-hour shifts during the day. By contrast working on the operational railway means:
- 4 hour window for night working (if you're lucky), but having to pay everyone for a full shift at overtime rates
- 1 hour of that will be spent mobilising and demobilising staff and equipment so the railway is handed back safely for traffic
- alternatively, weekend working which closes down the railway and needs to be planned with military precision (see Kings Cross last Christmas for an example of what happens if this goes wrong).
- specialist railway kit rather than standard civil engineering machines
- more staff time spent ensuring safety - sometimes exclusively (e.g. lookouts)
- massive 'interface' issues with existing equipment, e.g. piling through unrecorded signal cables
All this means that upgrade work sucks in more resource than equivalent work on a new build project, so when you have a limited pool of experienced engineers, technicians as at present, it becomes difficult.
Ditching HS2 wouldn't solving anything at this point. It's currently in the design stage with construction starting in 2017. The first years will be all about civil engineering which can be done by the wider industry. Only by 2022-23 (I guess) will P-way, signalling and overhead lines begin to be installed on HS2, which is when you might expect there to be competition for these specialist resources between HS2 and upgrading the existing railway. HOPEFULLY

the skill base will have been expanded by then to cope.