Can't enable new modern experience CSOM/C#

Question

I am aware of the scripts Microsoft and others have provided to enable the new 'modern' experience/UI in SharePoint Online. When I run the below code, however, the experience does not change. I am testing by visiting the site contents page of the root web.

public static void EnableNewExperience(Web web, bool enableRootCollectionFeature)
{
    ClientContext ctx = (ClientContext)web.Context;
    Guid siteFeatureId = new Guid("E3540C7D-6BEA-403C-A224-1A12EAFEE4C4");
    ctx.Site.Features.Remove(siteFeatureId, true);
    ctx.ExecuteQuery();
}

Answer 1

I am using the following PowerShell to change between modern and classic experience. This is working as expected and tested site contents and libraries.

#Script to enable / disable modern experience to site collection

$host.Runspace.ThreadOptions = "ReuseThread"  

#Required Parameters 

$sSiteColUrl = "<<site collection url>>"  
$sUserName = "<<user name>>"  
$sPassword = Read-Host -Prompt "Enter your password: " -AsSecureString 
$featureId = "E3540C7D-6BEA-403C-A224-1A12EAFEE4C4"

try 
{     
    #Adding the Client OM Assemblies         
    Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll" 
    Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll" 

    #SPO Client Object Model Context 
    $spoCtx = New-Object Microsoft.SharePoint.Client.ClientContext($sSiteColUrl)  
    $spoCredentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($sUsername, $sPassword)   
    $spoCtx.Credentials = $spoCredentials       

    ##Change to modern experience
    $featureguidSite = [Guid]"E3540C7D-6BEA-403C-A224-1A12EAFEE4C4"
    $spoCtx.Site.Features.Remove($featureguidSite, $true);
    $spoCtx.ExecuteQuery();

    ##revert to classic experience
    <# 
    $spoCtx.Site.Features.Add($featureId, $true, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None);
    $spoCtx.ExecuteQuery();
    #>

    $spoCtx.Dispose() 
} 
catch [System.Exception] 
{ 
    write-host -f red $_.Exception.ToString()    
}  

Answer 2

The problem was that we needed to enable the modern UI globally via the SharePoint admin center first. Then we had to opt-out of the new experience using code on our site collections where we did not want it enabled by ADDING the feature with the GUID "E3540C7D-6BEA-403C-A224-1A12EAFEE4C4".

1. We enabled the new experience globally in the SharePoint Admin center
2. We then ENABLED the feature E3540C7D-6BEA-403C-A224-1A12EAFEE4C4 at the site collection level for site collections where we wanted to DISABLE the new, modern UI. This allowed us to opt-out of the new experience.