Poly students can go universities, the only thing is
limits.
Good scores are needed to ensure a place. Even better scores are
needed if you intend to take something different from what you took
in poly.
That's for local.
For overseas, it costs even more.
There's no way we can build more universities. Land in Singapore is
limited. With aging population, it only serves to worsen situation.
Pavements need to widen for wheelchair-bound people, hospitals need
to upgrade in terms of size to serve more people.
As for A Levels, it's commonly regarded as a gateway to obtain a
university degree. I don't think there's a need to do away with it,
because A Levels offer different subjects from polytechnics and
ITEs. Since it's a gateway to university, these students will most
likely study either similar subjects or they won't. If they do
study similar subjects, then their knowledge will help.
IB (currently tested in one school?) could replace A Levels, but
the chances of it replacing A Levels entirely is probably
zero.
Your second idea is not a bad one, it's very similar to Australia
and other western countries. Those are known as community colleges,
which will serve as a foundation for universities courses if
students don't have the knowledge.