I think the Macedonian phalanx would have a greater chance against the Romans with the manipular system on flat terrain, but they didn't had the heavy cavalry to protect their flanks like Alexander did. To use more Macedonian phalanx and no heavy cavalry was cheaper but it made them much more vulnerable. Why didn't the enemies exploited this weakness earlier before?
You seem to be under the impression that the Macedonians gradually abandoned their heavy cavalry in favour of fielding more and more heavy infantry. I don't know where you got this impression, but it is not correct. At both of their famous defeats against Rome (at Kynoskephalai and Pydna) the Macedonians fielded thousands of heavy cavalry, and at both battles the Macedonian king fought on horseback. However, both times the cavalry was preoccupied fighting its counterpart in the Roman army, as often happened in ancient battles.