If the reason for cutting the route was that the flights weren't filling (although this is contrary to most reports) then they could solve this by putting on a 738.
If the reason was that they needed an A330 elsewhere, they could solve this by putting on a 738 - they have 60 + of these...
If the reason was that the business cabin wasn't filling, they could solve this by putting on a 738... You get the idea.
As long as the schedule remained similar to the current one, i.e. daytime flights, I don't think it would be a big issue to use a 737 on this route - it's better than nothing for sure!
I find it difficult to believe that Qantas is justified in cutting back PER-SIN from double daily to zero flights in less than a year.
Loyalty works two ways, Qantas. How can you expect your customers to be loyal to you when you are not loyal in return?