The argument shifts every time. Either the classic line will be reduced or it'll be improved. Different people make different cases, even in this thread!
The classic line will become part of an integrated WCML, i.e. it will contribute to services from and to all parts that it currently serves. The main difference will be that inter-city services, (as in London-Birmingham-Manchester-Liverpool-Glasgow-Edinburgh) will not clutter up the fast lines on classic routes. Those places that aren't served by the inter-city services will still get regular services in paths created by the removal of the inter-city trains. The express services that serve those non-HS2 stops will also be able to make stops at stations that in the last few years, have been unable to benefit from faster trains because the fastest IC trains have used that capacity.
Now, is how it might work: Anybody wanting to get to London from say, Lancaster, Blackpool or Preston would normally be advised to travel to Crewe for example and then board a HS2 train to London/Birmingham etc.. That's fine as even over that journey, end-to-end-travel times would be less they are today.
If a passenger for some reason chooses not to travel on HS2
(never, ever, use HS2, not a single inch of track or a square centimetre of rolling stock), they would need to continue south beyond Crewe on the classic route maybe to Stafford or even as far as Birmingham on a train that serves all of the stations that themselves need trainsn to get their passengers to London, (probably via HS2 from Birmingham interchange) so that passenger would them change to another non-HS2 train, probably stopping at Coventry, Rugby, MK, Watford Junction and eventually arriving in Euston. The journey would probably take over twice as long because the passenger didn't take the recommended route.
As I said, that would be like driving from Preston to London avoiding Motorways and taking the scenic route, something like this:
A6,A5185,A576,A5081,A56,A556,A553,A54,A34,A5148,A446,A45,B4429,A5,A5183,A5.
I'm sure that those local to South Lancashire could find better routes but my point is that travelling by non-recommended routes on roads or by rail and shunning infrastructure improvements will inevitably involve a much slower journey.