As to
1) &
2), Luigi addressed them in his QLUM 2017 talk.
Short summary: QuantLib
will evolve but using modern technologies is not a goal per se.
Finally, backward compatibility is essential (an it is there!). At least because in [big] financial institution quants cannot use what they want: they have to use what is conform with IT-policies.
3) Yes, there is always a better
a posteriori way to do something.
4). In 90% not necessarily in practice. And for the rest of 10% when it is, it suffices in 95% to call several instances of QuantLib in parallel (as separate processes). There are enough solutions for this.
5). ...
6). Yes, I mentioned this in my talk and have shown some especially tricky places.
However, my final remark was:
Luigi, keep up a good work to keep out mediocres from QL code!
Of course it was said as a joke but anyway, those who can (i.e. nearly everybody who really
wishes to) dwell into QL-code is welcome to use it.
For others there are QuantLibXL and Deriscope.