I'm going to have to backup a bit here... and re-state my position as I realize now what I initially said was not exactly what I meant to say.
Let me define first "critical mass" as being the time when product B overtakes product A and essentially wipes it out of the competition. VHS vs Beta was ultimately a 10-12 year or so battle before Beta officially handed in the towel, but the critical mass happened a few years earlier before Sony saw the light and threw in the towel.
DVD vs VHS is still technically going on because VCRs are still in the marketplace and manufacturers are still making consumer units... so we are in roughly year 10 of this battle. If things hold true to form, it may go another couple of years before manufacturers finally throw in the towel and stop making new VCRs. Of course part of this is due to TiVo and other PVR/DVR units, but that muddies the waters so I'll leave that out of this discussion.
Now... While the protracted Beta vs VHS battle was 10-12 years before Beta was formally wiped out... the actual overtaking happened before that, just as is the case now with DVD vs VHS, with DVD having overtaken a few years ago.
It also happens that DVD reaching "critical mass" happened sooner than VHS knocking out Beta so the statements about DVD reaching the controlling marketshare are true.
But what I was referring to was the time before that... When Beta and VHS came out, there was nothing else for the home user to buy... so immediately people who could afford either Beta or VHS bought one. But when DVD came out, VHS was a long-established consumer product so there was not the same drive for the customer to try a new product. DVD, aside from early adopters, sat around a bit in its first few years not getting a lot of shelf space or consumer adoption.
However, once people started really taking notice of all that DVD offered, it took off like gangbusters. So if we were plotting curves... Beta/VHS was a small spike then solid increasing volume over a long period of time... whereas DVD was semi-flatlined then a little growth then an explosion.
Folks were not resistant to VHS/Beta because they wanted this in the home... Those same folks were resistant to DVD because they already had a device that did what DVD offered, and DVD didn't let you record so you lost a feature! BUT once people took notice, DVD went nuts.
So... I believe it is both true that DVD took longer to initially adopt than VHS AND also true that it took over the market faster than VHS did.
Summarizing... VHS was adopted quicker as it had no opposition, DVD opposed VHS and it took a while to break-through... but once the breakthrough happened DVD penetrated the market much faster and more thoroughly than VHS did.
I hope that helps make my thoughts more clear and less conflicting with the sales data.
Interesting concept taking the VHS/Beta war and extending it then into a VHS/DVD war. While it seems to be true that DVDs are the fastest technology to go from zero to mass acceptance, you're points about replacing an older technology are intriguing.
But... just as you state that Beta lost years before it actually stopped, VHS could be said to have lost several years ago as well. When Blockbuster, walmart, and studios stopped selling or renting VHS recordings. I completely disagree that DVD vs. VHS is still ongoing; that war is definitely over. VHS is not going to regain popularity.
Just so, HD-DVD and Blu-ray is still an active war. While both claim to have won, neither really has done so--either still can make a sudden run yet.