I think your timeline and use of the word "many" are off a bit. GR/Riv started selling practical steel frames in '95. Take a look at a Trek, Spec'l or C'dale '95 catalog for a reminder of what mfrs were successfully selling in the US market, in total units of hundreds of thousands. Steel road was nearly dead, road bikes were built/spec'd like race bikes, and flat-bar hybrids were the primary choice for wider tires, fenders and upright riding positions. Sloping top tubes were still 4-5yrs off. Euro spec was dominated by "trekking" bikes, US flat-bar hybrids w/fenders/racks/lights. Niche primarily-steel players like Surly didn't exist.

Trek had three steel road bikes in '95, out of 10 total. The touring 520 stayed in the line because they had built it for years, in small numbers every year, never needed changing, why not? Spec'd w/28c tires. The 470 was spec'd w/23c tires, with: "...hot racing geometry for an aerodynamic body position." The entry-level 370 had a "Sport" designation, 25c tires, with "classic road geometry." Three steel road bikes out of 38 total models (22 ATB, 6 hybrid, 10 road). In my market the only steel road bike we carried was the 520, and the numbers were so small I wanted to cut it every year.

The type of bicycle GP advocated in '94-'95 didn't exist in the big mfr product lines and wasn't present on the LBS sales floor, which is where most people buying new bikes were looking. There were no discussions in product meetings about the need for steel drop-bar road bikes with quality tubing, clearance for fatter rubber and with fender/rack mounts. Mentions of GP at product meetings would draw chuckles and head shakes---he was that far off the radar. Keep in mind that mfrs don't build bikes consumers ask for, they build bikes dealers will buy. The "many" people you say were already hip to Riv issues were not walking into bike shops and asking for these bikes, so they were all but invisible to bike mfrs. People shopping the available road bike offerings didn't see any Riv-like alternatives, didn't know the alternative existed, so they didn't ask for it. Dealers didn't see the need since consumers weren't asking. Mfrs didn't bother spec'ing them because dealers weren't asking.

I don't know that Grant ever claimed or tried to convince anyone that he "singlehandedly" shifted the market. He did successfully make the case that consumers would buy practical bikes---though a niche market, it was viable and could be profitable. Reaching past the already-hip to the bigger numbers who only knew what they saw on the LBS sales floor was the key, along with offering product that made a practical build possible. And he was alone, until the ball started to roll and others jumped in.