Quite likely, yes... Long bus routes shortened or withdrawn,
more transfers required, including taking the MRT which would be
more expensive than buses. Previously there wasn't even distance
fare, so additional transfers would cost much more than just a
direct bus svc.
As I see it, dedicated rail lines (like the MRT, or the light
rail to be built along George Street in Sydney's case) provide the
capacity and reliability needed for a major traffic corridor, while
buses provide the directness and convenience to bring you exactly
where you want to go.
The issue is ensuring buses do not duplicate the MRT excessively
and inefficiently, while maintaining necessary bus links, along
short sectors of the MRT (connecting areas between the stns), in
areas not directly served by the MRT (eg Sengkang to hougang st 21
for 119, old 85 route), and where buses are faster and more direct
than the MRT (Express/Premium/City Direct).
Reliability of the MRT is becoming an issue in recent years, but
I doubt having more bus svcs duplicating the MRT (directly or in
the form of Express/Premium svcs) would help much. While they give
commuters another option, the capacity of the MRT (~300 pax x 3 or
6 carriages, 2-7 min headway) is simply much greater than buses
(~70 pax for SD, ~120 pax for DD/bendy, 5-15 min headway). In the
event of major train disruptions, buses are simply inadequate in
handling the huge numbers of stranded pax. I'm not saying we
shouldn't have duplicating bus svcs, but just that they don't help
that much when the trains break down.