Good point ... though I don't think anyone thought it would be almost 35 years after the Transitway opened before they started running LRT.
Though applying the same philosophy, we wouldn't be building the Eglinton line for future conversion to subway, or building subway anywhere near Sheppard.
I think LRT to subway conversion is a bit different than BRT to LRT conversion though. With BRT to LRT, the stations are usually smaller to begin with, and usually not underground. Therefore, rebuilding a station is a lot easier. BRT also has the advantage of being rerouted onto a parallel route while the upgrade is being made. With the case of the Ottawa LRT upgrade, the buses will be run on Scott St while the upgrade to the Transitway ROW is taking place. This sort of parallel diversion can't happen with an LRT, at least not very easily (in most cases, it would be replaced with a bus route while the upgrade takes place).
But yes, I don't think anyone thought it would take this long for the BRT to LRT conversion to happen. In a lot of cases though (and I think this is the route a lot of 905 municipalities are taking) is that they're building a lot of BRT for relatively little money, and using it to grow ridership. In places where transit ridership is low to non-existant, building 2 BRT lines in a lot of cases is better than building 1 LRT line, even though they would end up costing the same. Blanket the area with BRT service, and then when ridership on certain routes grows to the point where LRT is justified, the upgrade can be made with only some disruption to the existing BRT service.
With your mention of Eglinton and Sheppard, you know that I do not favour LRT on Sheppard, nor do I think that a subway extension on Sheppard (short of a minor extension to Vic Park) should be anywhere close to the top of the priority list. On the contrary, I believe that using the model that I described above would be the best medium-term strategy for Sheppard. Put in dedicated BRT lanes now, and grow the ridership to the point where LRT or subway could actually be viable. For most of Sheppard, especially east of Agincourt, the ridership barely justifies BRT, let along LRT or subway. Starting off with BRT would be an inexpensive way to get reliable transit to that area, without breaking the bank. Use that extra money and spend it on areas that are in urgent need (like Eglinton, where the ridership to justify higher order transit already exists).
For Eglinton, I'd be more concerned with station expandability (ie going from 4 car to 6 car trainsets) than what technology is used. If the stations can't be expanded easily, then there's problem with the design. But from everything that I've read, station expandability won't be an issue.