Given that aviation is changing from hub-&-spoke model to point-to-point model, there is a risk that Changi T5 would be under-utilized.

But I believe SG government already has plan B if this happens. Maybe they would demolish the existing T4, and move the traffic to other terminals.

Yes, point to point model has existed for decades. But its weight has been growing in the last 20 years.

A few factors explain this:
1. Budget airlines favor point to point.
2. Newer aircrafts such as A350 & B787 could fly further than the older models.

Starting next year, Qantas would offer non-stop flights from Sydney and Melbourne to London, bypassing SIN or Middle-East.