Changing the M-DCT oil at 50k miles seems like overkill to me but I guess that it depends on how the car is driven but even then.
Normally (and I say 'normally') BMW should have replaced the engine oil + filter, rear diff oil and transmission oil (+ clean filter) for the MT after the break-in period (2000 km / 1200 miles).
BMW replaces the rear diff oil and the transmission oil (manual transmission only) + clean the filter after every 5th engine oil service.
This means that when the car is driven normally (and I say again 'normally') that the rear diff oil and the transmission oil for the MT is going to be replaced when the car hits the 150k kilometer mark, which is rougly 93k miles (or after 10 years).
The M-DCT oil is not replaced as far as I know because it is actually a 'lifetime' fluid (
BMW DCTF-1 Pentosin FFL-4), unless you explicitly ask for it and I think that it will still be refused. It is also unclear what BMW means with ‘lifetime’.
A lot of techs will have a hard time finding the correct procedure for the F8x and human errors can occur which will result in actual problems with the DCT. I think that the 5th engine oil service (or after max 10 years) is a good mark for replacing the transmission oil of the MT or DCT for ‘normal’ driving. Don’t underestimate the cost which might be 300 to 400 euro (9.5 liter of fluid + labor)
I remember reading several threads on different forums (E92 M3) about people that track their car and there is someone who even sent a sample of the transmission oil to blackstone labs after about 30k miles and after several hundreds laps on a circuit and the analysis showed that there was barely wear. I have also read posts about the transmission oil still looking clear after 100k miles, ...
I personally follow the BMW CBS which is an oil service interval for the F8x of 30k kilometers / 2 years and I replace the oil and filter in between these service intervals (15k km / 1 year), oil is still clear. After 120k kilometers I will have the DCT fluid replaced as well. Taking into account that I drive 25k kilometer / year this means replacing the DCT fluid after a little less than 5 years in my specific case. My lease ends at 125k kilometers / 5 years (November 2020) and I am planning on keeping the car but the G8x will become my next DD.