Also saw this covered in dezeen - https://www.dezeen.com/2020/07/28/autodesk-revit-bim-software-criticism/
To me it feels like super simple math - spend a percentage of in-house development resources to an open source contribution and everyone benefits. London is my home market, so I know for a fact that many of these companies spend on developing their own tools on top of Revit due to Revit's lack of features that are deemed essential for design.
I have no doubt that Revit will be replaced the way that QuarkXPress became replaced by InDesign and Adobe is now gradually starting to be replaced by Affinity Suite (at HOK, we're making the shift due to increasing Adobe fees and inflexible pricing, also many others on the list of the open letter are either partially or fully thinking about shifting to Affinity from Adobe).
Archicad seems to be developing rather well and their own take in using GH for node-based scripting is great (as opposed to building something on their own like Dynamo). It would be tough to replace Revit as it is an industry on its own - BIM Managers, loads of plugins, requirement for submitting bids, and Construction and FM usage. But, at some point companies will come around to realise that their money (or at least of some of it) could be better spend on trialing alternative software and getting the necessary infrastructure in place for proper project usage.
The trouble in medium to large companies will be office fragmentation. At the moment, Revit serves as the de facto BIM/CAD standard so if a project within a company needs more resources, it is easy to allocate more people. Whereas in an office where some use Revit, others Archicad/Catia/Freecad/BlenderBIM, it would add an extra level of complexity to managing office resources. However, this is not an excuse and the nimble and adept companies are happy to have some staff that are more specialist in some software than others.
I can certainly foresee a future where Archicad replaces Revit for smaller to medium projects and Catia replaces larger projects. It would be great if in this "inevitable" shift, Freecad and BlenderBIM could grab a percentage of the AEC market.
Having invested a significant amount of time in learning Freecad, I have no doubt that if the signatories of the open letter invested 5% each of their R&D budgets to help refine and develop new tools alongside the current Devs, Freecad could be user-friendly enough and have the right toolset to be used on smaller medium projects as a start (where collaboration between in-house team members and out-of-house consultants would be necessary). Probably the same with BlenderBIM?