I suppose the delaying of the remaining vintage based characters is just like any line does. They spread out demanded figures with other stuff to maintain interest in the line.
I'm sure that's how it started. But like canprime pointed out - I also think a lot of those customers they were trying to drag along with them chose to move on anyway. Mattel needed to walk that line between giving fans what they wanted, and stringing things out to keep them invested. The reality is that they just did an absolutely terrible job of it and dragged it out so long that plenty of people just stopped caring.
Obviously it's a VERY different beast, but I think it's comparable to Star Wars. You can take a couple of years to get the entire main ANH cast out there in figure form, depending on how many releases you have per year. Wait too long and customers are just going to catch on to what an unserious dickhead you are and either lose interest or literally refuse to buy your stuff out of spite. Mattel is just an insanely-poorly run company that makes awful choices all the time.
It's unfortunate that the other side of this argument rarely gets any support from big name companies, but it's something NECA did really well a few times: Get everything people REALLY want out there as quick as you can. Show them how committed you are to being customer-friendly and fan-oriented. Hook them. And then drag them along once they're hooked.
People didn't stop buying Cartoon TMNT once the main cast was done -YEARS- ago, right?
JADA has done this with Street Fighter and I honestly don't know if any toy company currently has more goodwill with customers than they do. If they got the license for Mortal Kombat tomorrow - everyone would be there for them, fully trusting them to get every character done eventually and, just as importantly, to make good on the commitments they make to the customers. Mattel has literally NONE of that goodwill and they did it to themselves.
If I were at Mattel, I would have argued a long time ago that the best sales pitch for Sharella is 'look how deep we have to go to find characters to make because we keep making everything you asked for' and the WORST sales pitch for Sharella is 'look, how about this instead of the figures you have literally begged us to make for several years?'