They provide gtag as the default since Google’s hope is you will use multiple Google services on the same page, and using gtag.js facilitates that use case the easiest.
Gtag to be very clear is not a replacement for analytics.js. It is simply a convinience (for developers) wrapper around analytics.js because it makes the output in nice JSON.
It does not support the full functionality of analytics.js because it is not a complete wrapper.
Moreover as it is a unique query string request, by using it you are requiring visitors to your site to load an extra JS file that will then load analytics.js, and unlike analytics.js, gtag is not going to be in their browser cache because of that query string.
There are more downsides, but basically the bottom line is this, unless you are using multiple Google services, and not doing anything advanced with them (that the wrapper doesn’t support) you should not be using gtag.js. As MonsterInsights and ExactMetrics only do analytics, it doesn’t make sense for us to use or recommend gtag at the current time.
I’ve talked to a lot of people at Google about putting it back, because obviously with us supporting both MonsterInsights and ExactMetrics, their future plans are always of interest to us.
To quote Philip Walton for example (since it’s a public comment I can reference), the lead of autotrack.js at Google,
> I agree that it shouldn’t have been removed from the installation instructions in the Google Analytics admin section, and I’ve filed a bug about that internally. Ideally, there would be an article in the help center clearly explaining why there are three different options, and when it makes sense to use each option…….I know of know of no plans to deprecate analytics.js
If and when they want us, the largest GA partner on the planet by number of installs, to migrate over, we’ll be the first to know, and that will be communicated explicitly with all of our users, and we’ll handle the migration.