All you do is increase the inefficiencies and create pinch points in different places. You seem unable to comprehend why that's a problem.
It's not a
new problem; it's not making a problem that doesn't exist, it's merely emphasising a problem which exists already - lack of capacity at critical points.
Any increase in potential capacity elsewhere will encounter the same problem, of needing to get more trains through such critical points. That cannot be allowed to stand as an argument against works to improve capacity on routes feeding those points, otherwise we are forever stuck with the overall capacity arbitrarily restricted to whatever limit the present state of those points imposes. Sorting them out is a need imposed by any kind of intention to increase rail capacity and usefulness, not something unique to a particular proposal.
The original builders of the railways of course also had to deal with these same problems when they put a new line in - they did
not invariably consider a new line as an isolated entity, a kind of optional extra which they could simply "drop in" to an unmodified existing network and have it Just Work like sticking more RAM in a motherboard; rather they considered it as what it was - one part of a generalised aim of improving their capacity, of which other parts included the improvements needed elsewhere to allow the new line to be properly useful. It may have taken them quite a long time to get all the parts sorted out, and in some cases they never managed it, but they did understand that it was something they needed to do. The long-drawn-out saga of the Midland's various works to improve their capacity to Manchester, of which the Peak route is an episode, is a noteworthy example.
Because the original railway builders did deal with these problems, in very many cases the reason we are now encountering them again is simply because some divot ripped all the original solution out again and rogered the capacity, merely because they didn't happen to be running very many trains over it at the time so they figured they could destroy everything and make sure they could never run any greater number of trains over it in the future. So in their present form, the problems are artificially created; and the solution is obvious, though admittedly there are also cases where it has been artificially made harder to carry out by some complete and utter cretin flogging the land off to be built on and make double sure they'd permanently destroyed it.
The Manchester area is a particular example, where the infrastructure was gradually and painfully built up to overcome the bottlenecks and limitations that were encountered, then much of it was ripped out in an orgy of destruction which introduced new bottlenecks in what remained, and now the place is bursting at the seams with constant calls for more capacity in all directions but not enough room for any new trains to go when they get there. The old Midland network is in similar case; a system which once provided four or more tracks (via four-track main lines, more than one route, or both) to all the major population centres along the northern half of the M1, plus Manchester, can now only provide a fraction of the service it ought to be capable of, not because the trains can't go fast enough - they're fine once they can get going - but because things like spartan and threadbare junction and station layouts, and reduction of multiple tracking, won't let them get out of each other's way well enough. These are not inherent problems which preclude any useful reinstatement of the Peak route; rather, they share with the Peak route a common membership of a set of problems which already need to be solved, with the aim of increasing capacity and restoring capacity which has been lost, and by comparison the Peak route is a fairly minor member of that set.
And if a new freight line is needed, the find a decent formation, not a rehash of a suboptimal Victorian one with gradients, curves which goes through tourist areas.
Like the Settle and Carlisle...?
The worst gradients on that route are all on the bit that still exists, up from Miller's Dale over the top at Peak Forest and down the other side to Chinley, which is nearly all 1 in 90. So the freight trains are already having to cope with the worst gradients on the route that they'll ever encounter, no matter what happens; and since Peak Forest is where the stone traffic comes from, the loaded trains are going in the easy direction.
The gradients on the closed section are mostly around 1 in 100 - which is the same as the ruling gradient on the Hope Valley line, and the gradients turning left at Chinley aren't much shallower, with a lot at 1 in 100 or 1 in 132. So basically it makes no difference whichever way you go, closed or open. Considering Hindlow as well, it actually makes things slightly better, since trains from Hindlow to the south would no longer have to thrash up and over Peak Forest summit to get out as they do at present.
Similarly with curvature - all the surrounding routes are also "Victorian routes with gradients and curves". Indeed, with a handful of exceptions like Brunel's GW all the routes in the country are Victorian routes with gradients and curves. I can't take that seriously as an argument that such a route wouldn't work when we have such an extensive degree of demonstration that it does.