First, cathedrals were not built in order to earn a financial return.  The rate of interest was probably not very relevant. In fact, estimates of medieval interest rates place them generally higher than modern interest rates.  For a reference that is available on the web, see

http://www.ecn.bris.ac.uk/www/ecesc/Articles/mn.pdf

Second, low/negative interest rates can also stimulate consumption, by making saving less attractive.

Third, even if money has a negative interest rate, other investments will not (and if they do, we don't really want people making them), and it will still be desirable to allocate your capital to those investments that pay off more and sooner.

It is also important to remember that the future is uncertain, and the far future is very uncertain, and investments tend to have finite lives (and/or high maintenance costs).

I think it is hard to say what investments people with then-current knowledge and a very long term view should have been making in the early 20th century that we would be glad they had made now.  For instance, this is the 50th anniversary of the very large investments made in the US Interstate highway system. Certainly that was a far-sighted investment, but in light of the liquid fuels crisis which is now in view, perhaps not far-sighted enough?

I'll stick to conventional discount rates, I think.