At the time I envisioned closing Pancras Road and putting a new roof over the space linked to St Pancras, so in my opinion, even better.
--- old post above --- --- new post below ---
Maybe at this exact moment in time, but these are big changes that will last long after the current timetables.
The Thameslink core will top out at 24tph. Moorgate can only turn around so much, and the platform lengths are limited so if longer trains become required, you lose Moorgate. Through simple virtue of the fact that two two-track railways will merge at St Pancras capacity is going to be wasted as you couldn't increase the service on both the MML and ECML slows to 24tph if required to handle future growth.
If you need more capacity on the MML slows, where can you send the services if the Thameslink core is full? St. Pancras has no platforms spare. There are no handy branches a-la Moorgate. Where would they go? The only reasonable option is to divert paths from the ECML services which do have alternatives, be they KX, Moorgate, or something else.