The pop up method was already used as default by many users, but the users who had problem using the new method was able to use old compose method till now.
Gmail has been under quite a controversy because of its targeted advertising, using the keywords in the private email and tracking a advertise for it and showing it to users. This experience has been annoying for many users because it is thought that emails are not anymore private and are sold to advertisers bidding for keywords.
Also, Gmail changed its look from simple to tabbed views which makes easier for users to see which are messages are really important, which are social messages and which are just promotional emails also considered spams.
Google team is working on creating more and more efficient tools to assert its dominance and to comply against the European Union ruling for greater transparency.
Soon this, pop up method, will be the only way to compose a message as the company rolls out the changes to everyone in a few days.
Image Source : AliceKeeler