I disagree with you about MLS and capacity.
When the viz or cloud base gets close to Cat 1 airports need to go to LVP’s. This protects the ILS signal and allows safe autolands. One of the protections is to increase the spacing between aircraft on finals and to restrict when departing aircraft can pass the CAT II hold. This substantially reduces the flow rate, particularly at single runway airports.
When the viz get very bad restrictions needed for safe ground movements become the limiting factor. BUT when the viz is 500meters it’s the ILS protection that limits flow rates.
The current straight in approaches used with MLS in met just below Cat I (which is what we have most of the time LVP’s are in effect) would get a lot more Pax to their destination on time. Most airlines in Europe would be willing to pay for this. As for GA, well at the main airports in Europe its not a factor.
As to equipment and training it’s a non-issues for straight in MLS. Most airlines dealt with the issue of FM immune ILS and VOR with “multi-mode” receivers. These just need a MLS card putting in the box. The operation is identical to the crew. So the training should not be much of an issue. You are right about the need for all to have it, but as with RVSM and 8.33 radios there will be a date. Then after the date you will not be able to go to MLS airfields if you don’t have the kit