The down side of short connections is that some people might actually prefer a longer connection on a long distance journey to have a break/coffee/sandwich/fresh air.
I recall and still enjoy the thought that the journey might actually be relaxing and enjoyable- so no need for as break; a coffee and/or sandwich available on the train (and consumable seated in the buffet car); and fresh air at New Street? !
Another downside of short connections- when one train is delayed. I seem to recall an attempt to place connections at New Street at the heart of matters with the introduction of WCML electric services back in the day. OK if/when all ran to time. What to hold if not, losing connections further up or down the line.
Maybe with 2tph each way A/B and C/D and thus trains at e.g. xx00, 15, 30 and 45 with min 12 min connections it would be possible to arrange something reasonably acceptable and regular for XC journeys like Manchester/ Newcastle > < Bournemouth/ Plymouth?So which journey pairs are you prioritisng there? As I said above the northbound south coast services arrive at xx.50, a NE/SW goes at xx.12. That is a connection. The northbound SW service doesn't connect into a Reading/Bournemouth because it doesn't need to. Cheltenham to Banbury and Oxford is a tiny flow and better served via Didcot and International and Coventry you would change for an Avanti or WMT that does connect? The same applies to the southbound SW services, what journey pairing are you trying to do that doesn't connect into a WMT, TfW or Avanti service?
What you are asking for is for New St to be hamstrung by forcing cross platform connections which can only be done on a limited amount of platforms anyway?