The modern UI in SharePoint differs quite a bit from the classic view. Many users prefer to switch to classic view when viewing lists and libraries. Some organizations even configure lists and libraries to use the classic view at the tenant level. If you are one of these users, it is time to stop doing this and get accustomed to using the modern UI. There are several reasons for this. One is that all new SharePoint functionality Microsoft is provisioning is primarily built for the modern UI. While the classic view is still supported it will not contain as rich a feature set as the modern UI. Take for example, PowerApps and Flow. Want to create a PowerApp or Flow in classic view for a SharePoint list while viewing the list, the process to do so is not intuitive at all. In the modern UI, you see the options to create a PowerApp or Flow, right next to the new list item button above the list data.
If you are worried that you have customizations that may not function in the modern UI there is a tool built to assist. Microsoft has created a SharePoint modernization scanner to help facilitate the move to the modern UI. You can download this tool, connect it to your tenant and let it run. It will identify items it sees as potentially being problematic in the modern UI.
If you still aren’t convinced, note that Microsoft as of April 1, 2019 will not allow you at the tenant level to only use classic mode. After this date, such lists and libraries that were configured that way will be automatically shifted to modern. In the event you feel you still need to keep a site collection on the classic view, you will be able to via powershell disable the modern experience for that site collection.
If you are interested in further discussing the modern experience or other SharePoint topics, please feel free to reach out to us via email.