Putting my (non-existant) MBA hat: ACS will never be able to compete on price (i.e., the price you have to pay for hosting ACS site) with low-end solution based on PHP. It's as easy to bring down shared PHP/MySQL box as it is to bring down shared ACS box (last time I've heard you don't really have to try to bring MySQL to it's knees). However, PHP has no overhead, ACS has quite big overhead per page request (request processor, database hits for permission checks). Low-end solution can obsolete ACS in the long term by adding more high-end solution. Given that ACS cant' compete on price the only way (long-term) to compete is by adding more high-end features that are not easily replicated in other toolkits.
