I think you misunderstand. Generating images ahead of playback was never a viable option. It's mostly too slow to be done on the end user system. YouTube does it in their cloud, but that only works because video are centrally uploaded to YouTube. For private videos, this needs to be done by the encoder software.
The way this could have happened a decade ago is that popular open-source encoders would have agreed how to store the thumbnails in MKV. Now it's frankly a little late.
Thank you very much Remy for your clarification. I understand your scenario much better now with your message quoted above. It makes a lot of sense as why you say that it is a "little late now" for the strategy you explain to be viable today.
You have also very good point when noting that "Generating images ahead of playback was never a viable option. It's mostly too slow to be done on the end user system".
I guess on my end, what I am just trying to say is that it would still be very useful to Generate images ahead of playback on my own end user computer. It is a very slow process. I fully agree with you.
However, at least on my use case (which may or may not match other users use cases), I could perfectly for example leave my computer generating the images for a number of videos during night time for a period of 12 hours, and then have the images ready only the next day. I do not need the images to be generated quickly, and it would still be an amazing feature.
I could use FFMPEG or any other tool, without even VLC having to do the job of generating the images, just VLC providing the user any way to link the images, once the images have been fully generated by any external tool. (I noticed that VLC already has the ability of generating thumbnails, but as of now only at the current point of playback).
Thank you very much again for your great advise and time on this discussion. Very highly appreciated. My apologies if I'm still misunderstanding your information.