Re: And for those lost episodes.
What pays for the animations is the expected number of DVD sales and downloads.
Which isn't much. Hence most of the investment in animation is to fill in between existing and recently recovered material that hasn't had a previous outing on DVD. Eg Invasion, Ice Warriors, Tenth Planet.
And those were stories which were expected to be popular. By comparison Underwater Menace was audio and stills standing in for missing episodes.