Sure, road traffic may well still make up the majority of trips to and from the airport, as it still does for Manchester despite it having had a station for some years now and the political imperative that near enough anywhere and everywhere in the North has a direct link to it. Like I said, I would envisage a station at that location (Hale Rd/Western Ave) to have probably more local traffic from Speke than from the airport. A tram-train with one or more intermediate stops would have even less dependency on airport traffic.
The buses may well 'cope' well but buses are slow and less reassuring to someone from out of town who may not know where and when to get off, or even if they're on the right one (especially those from a foreign country). In these situations, a taxi, although more expensive, is far more reassuring, as well as quicker. A fixed link in form of a rail line that goes to the main rail station in the centre would be another option for those heading into the centre. Sure, many will not contemplate public transport over private cars or taxis no matter what, especially those whose final destination is away from the railway line but there would no doubt be a fair few who would consider the rail service in a way they wouldn't consider the bus.
