According to the latest schedule, as of October 16th, 2018, any users who’ve opted out of the new Gmail will be automatically migrated to the new experience.
They won’t be able to opt out after that date. Users who’ve already transitioned to the new Gmail will lose the ability to opt out as well.
I’m reminded of the adage that the only one who likes change is a wet baby.
So, why is Gmail changing?
- Increased security
- Updated code
- Better integration with other tools
Personally, I like the new interface. It provides some good tools to increase efficiency. However, I understand that not everyone feels the same way. Increasing security is an extremely important consideration at this point of the Internet development. You can choose to switch back and forth between the versions until October 16th. Then, the decision on which to use will be easy 
.