IMO this relates to who the end consumer of websites ends up being. All the arguments for just using MCP ignore the reasoning that agents need based on contextual clues (e.g. if you're booking seats at a movie theaters, you'd want to choose based on personal preferences and taking the movie itself into account) and treat the website itself as the first class product.

Developing MCPs only works if the capabilities for the site don't involve any iterative reasoning and acting for the user, which things like travel and dining do :p