Google has undoubtedly decided to make this change to a mobile first index because we are “living in a mobile world” primarily and it makes sense to make this change.
If you have a standalone mobile version of your website, which many firms still do, almost certainly this will have been cut-down version of your site. These cut-down versions will have fewer pages and shorter pages, neither of which is good for SEO. So if you have one of these sites and dominating Google is important for your type of firm, then you need to do something about it!
Google is fully splitting its search index into two distinct versions: a rapidly updated mobile one, and a separate, secondary search index for the desktop web.