Firstly, you’ve missed a key connection - OOC will be connected to HS2, which will allow vastly improved connections to the North West. Don’t forget that the current opposition to HS2 to Euston is largely Rishi’s pet project - he’ll most likely be gone in a few months time.
Travelling from South West or South Wales to East or North East via London might make sense, but travelling to North West makes a journey L shaped.
There is a fairly direct service (even if it is currently often a mere 4car voyager). The notion that everyone will suddenly think let's do extra 100 miles and change at Old Oak Common instead of taking a more direct train (often without a change) is bonkers.
If you were driving from Devon to North West wouldn't use A30, A303, M3, M25, M1, M6 for the fun of going near London, instead of A30, M5 to M6, so why would anyone think taking an indirect train via west London would be anyone's sensible choice.
Probably not even going to be a time saving if HS2 phase 2 doesn't get built. And if it does eventually open in late 2030s the voyagers will probably have been replaced by then, so on board experience going direct might be better than travelling on a GWR IET and a DfT spec'd HS2 train.
Old Oak Common will never be a key connection, it doesn't work if need a train from Kings Cross or St Pancras, and is indirect if train starts at Euston, basically people from South West would much prefer a quick connection at Birmingham Interchange, OOC as an interchange from SW its located 100 miles too far towards South East
Bristol to Birmingham is about 95 miles
Bristol to Birmingham via Old Oak is about 210 miles
exact distances depend on route