It was the DfT that decided there were to be no TfW trains to Bristol. Lack of capacity pre-Filton requadrupling and East Junction remodelling was the reason given.
They did promise to revisit that position when the upgrades around Bristol were finished, which they now are, sort-of.
But that was when Filton-Temple Meads was supposed to be wired (and Cardiff-Swansea).
I think it's fair to grumble about the transition from the old franchise to the new, but it was the franchise letting regime, not the incumbent operator, which crippled the rolling stock replacement options.
There was no prospect of varying the Arriva franchise terms, or of WG coughing up their own money for quicker replacement before franchise expiry.
Remember the money all comes in one form or another from Westminster, allocated via various transfer mechanisms to Cardiff Bay.
The Pacer exodus was posted several years in advance, and clashed with the TfW franchise replacement, but both governments chose to ignore it till it was too late for replacements to arrive before the Pacers had to go.
Northern was in a similar position a couple of years ago, but have turned things round with the new trains visibly improving the service.
TfW have to do the same over the next couple of years, but they are currently in about the worst place possible, resource-wise.
The key thing is getting the new trains out of the depots.