ProAc has made a living off basically doing that.
Why do you think ProAc are using off the shelf drivers? I am not familiar with the products but a glance at their website shows many drivers with Pro-Ac written on them which would require a driver built to specification rather than an off-the-shelf driver. Some of the drivers are non-standard colours for off-the-shelf drivers. They would appear to be drivers built to ProAc specifications by various driver manufacturers. It is possible that the easily customised parts like magnets, coils, formers, spiders, cones, etc... are the same as off-the-shelf DIY drivers but it would be odd if the driver manufacturer is having to gear up to produce batches anyway for the cosmetics.
And they are arguably better than PMC.
It will depend on what you look for in speakers. Both companies have been around for a while and likely have a bunch of happy customers which is perhaps the most important criteria to the market. Neither seem to be players in the high technical performance for reasonable cost that is of most interest to me but that doesn't make them bad speakers just inappropriate ones for my interests.