On the PDP architecture:
I think that Infocom’s history of sunsetting systems (TI, for instance) suggests that they did understand that no platform lasts forever. Certainly they would have seen signs of this in 1985, when the first IF plus game, AMFV, shipped.
The struggle to maintain compatibility with C64 games would be another indicator that the platform was running out of gas. Implementors have talked about that limitation over the years. I most clearly remember Amy Briggs talking about it with regard to Plundered Hearts. By 1988, the value of the technology was quite questionable. Zork Zero did not ship simultaneously on Mac and DOS (five months apart!), despite being written for the Z-machine. The value of technical planning and R&D is that some preparations can be made for these eventualities.
I can’t find 86-89 sales data by platform in the Infocom cabinet, so I’m not sure what the trendline of C64 was, sales-wise. If someone else has the data, I’d love to see it.
So far as I know, Cornerstone was not made or tested by the games division in any way (someone correct me if I’m wrong!), whose sole released platform was MS DOS. The Cornerstone team consisted, I think, of over 100 employees. There’s no telling why the team didn’t raise flags over performance (or maybe they did), but I don’t think C64 performance with IF games played a role.