OK, how about this hypothetical explanation:
The A5 SRBs add a lot of fixed costs and perhaps variable costs. Removing the solids will obviously reduce payload, but most payloads do not need the full payload capacity anyway. In fact most launches are dual payload launches. Using more launches will not affect the portion of fixed cost allocated to each payload.
Removing the SRBs also removes an enormous amount of thrust. In fact, according to the data on Ed Kyle's Space Launch Report
the A5 core would have a T/W <1. By reducing the size of the stage and increasing the thrust of the engine you could get back to an acceptable T/W ratio.
The basic configuration of A6 would then satisfy European institutional needs. Heavier commercial payloads would require strapons and apparently the extra specific cost of these is lower than those of the current SRBs.
All in all the plan would seem to be to get lower fixed costs and as a consequence lower subsidies to commercial payloads while still being able to remain competitive with emerging launchers. Any cost sharing the solids had with French nuclear missiles would be taken care of with Vega, which is perhaps expected to be economically viable without too much subsidy.
The above is all conjecture, it would be interesting to get an expert opinion on all this.