MAS may need to shift all its operations from Subang to KUL once the ERL opens. That's what the government says! Air Asia needs to shift to KUL as well, but they are currently trying to negotiate with the DCA to continue their operations at SZB. Air Asia said that their fares would not justify their passengers traveling 45 miles to KUL just to catch a cheap flight. The cost of a return ticket to KUL is about the same as the cost of a one-way ticket to Penang etc. Then, I would had thought that Air Asia might fail in that sense that people would rather travel by coach then.
Normally, low-cost airlines would operate at a secondary airport instead of the primary ones. Eg. Easyjet at Gatwick and Luton instead of Heathrow, Ryanair at Stansted, Brussels-Chaleroi, Frankfurt-Hahn instead of Heathrow, Brussels-National and Frankfurt-am-Main respecitvely. But I would seriuosly not fly out from Frankfurt-Hahn though, heard that it's something like 100 miles away from downtown Frankfurt.
Plus, Air Asia mainly operates on a O&D traffic. MAS operate where they are connecting passengers to these cities from other international cities. Like here in the UK, when you purchase a ticket to Australia/New Zealand, MAS will give you a free side-trip to one of the MAS national destination or Singapore. Thus, this will enable MAS to sell it's national destinations to non-Malaysians, who need not worry about getting to and from the airport since they are only transiting at KUL.
Since Air Asia mainly tragets Malaysian, many people around Kuala Lumpur would actually prefer to fly from Subang instead of KUL. This same goes for people who plan to visit Kuala Lumpur. If I am not mistaken, the landing approach to SZB is much better than KUL as well. All you see in the approach to KUL is trees, although you will overfly SZB when you are flying in from Europe.
BTW, yes, I am a Malaysian, but currently residing in UK right now. Going to reside in America later this year.