I would say the amalgamating of DC pensions is a bit more complex than you suggest. Firstly you need to understand if there are any special benefits attached to them or penalties for transferring. But it's not necessarily cheaper (or better) to have them all in one place, unless that one place charges lower fees (or has better investment options). And sometimes, different pension pots give you access to different types of fund, for instance. One may be low cost with a fairly limited set of options, another might cost more but give you more scope to choose (for instance) adventurous investment options which you might want to do with part of your retirement fund. Transferring a work DC pension to a SIPP will even allow you to buy individual company shares or investment property with your funds. So it might make sense to keep pots separate to spread your bets with a few different options. You can move pots around at any time, including once you're retired (unless you take an annuity), and usually without penalties (though you should check).
However, if you identify a particular pot you have that is clearly poorer value than another one (eg with worse fees, or poor investment options available through it), it makes sense to combine those with a better one you have (or even just open a SIPP separately, which does the same thing effectively). Or at least choose a different investment approach within it, that might have lower fees or better likely returns. Some might want to get advice about that, or just take the plunge themselves if they're confident in the decision.