Having a website developed and paid and now I get this message ?



I would seriously like to know when an MS platform is EVER a true advantage over an OSS-based solution for creating a website.

Some thought here. LAMP is the industry standard, Wordpress is all PHP/MySql, the best ecommerce frameworks are all written in PHP/MySql.
Facebook is still developed in PHP.
A lot here talk about php is cheaper than Asp.net, but this doesn't mean that is worse. Thousands and thousands developers contribute to LAMP with added features, bugs fixes, libraries, classes, extensions.
Asp.net is only managed by MS and I wouldn't use it even if free.