I would argue the combination of technology and the legally mandated reduction in the number of points-of-purchase enabled the AMC users to exploit their increased access to the larger number of SFR appraisers.
(More supply + less demand during a slow RE market) * easier mass shopping = crab bucket competition and lower fees.
The CGs can be considered the control group. Same technology and the same easy shopping for fee and the same prohibition against using MB-engaged appraisals. The only difference was the lack of gross oversupply for the lenders and AMCs to exploit, despite the technology making it easier for them to shop for fees. The CG fees still dropped via competition but not enough to starve any of them out of business. As demonstrated in the numbers: 3489 in 2001 reduced to 3174 in 2018. Barely -9% and at least some of that might be primarily due to age, not starvation.
In yr2000 fee appraisers made cold calls and sent faxes to individual loan originators. Contacts maintained in a rolodex. Calls received on an answering machine or by a live admin asst. Or on a beeper. It was one step up from going door to door. The internet, email and the online databases of appraiser contacts changed all that. The inefficiency of selling appraisal services in person was always going to be outcompeted in the market by the increased efficiencies of using email and portals. That much was inevitable no matter what.
Then DF amplified the effects of an internet-enabled marketplace by reducing the majority of the eligible purchasers of appraisals to just the lenders and their designated AMC lackeys. Appraisers are now marketing to maybe 1000 potential buyers instead of 100,000 outside brokers.
We can say that DF didn't have to happen, and that fee appraisers could still be free to sell appraisals door to door but the impetus for DF was the same as the impetus for FIRREA wherein the regulated lenders were prohibited from using appraisals that were engaged or controlled by the outside and in-house loan originators. DF just brought the SFR trade under similar restrictions that had already existed for many years on the FRT side.