No need for a specific namespace replacer it seems sourceName prop is used for that

build/templates/UmbracoProject/.template.config/template.json

@@ -29,7 +29,7 @@
             "continueOnError": true
         }
     ],
-    "sourceName": "UmbracoProject",
+    "sourceName": "Umbraco.Cms.Web.UI.NetCore",
     "symbols": {
         "version": {
             "type": "parameter",
@@ -38,16 +38,6 @@
             "description": "The version of Umbraco to load using NuGet",
             "replaces": "UMBRACO_VERSION_FROM_TEMPLATE"
         },
-        "namespaceReplacer": {
-            "type": "generated",
-            "generator": "coalesce",
-            "parameters": {
-                "sourceVariableName": "name",
-                "defaultValue": "UmbracoProject",
-                "fallbackVariableName": "name"
-            },
-            "replaces":"Umbraco.Cms.Web.UI.NetCore"
-        },
         "PackageTestSiteName": {
             "type": "parameter",
             "datatype":"text",

build/templates/UmbracoProject/UmbracoProject.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
     <PropertyGroup>
         <TargetFramework>net5.0</TargetFramework>
-        <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">UmbracoProject</RootNamespace>
+        <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">Umbraco.Cms.Web.UI.NetCore</RootNamespace>
     </PropertyGroup>
 
     <ItemGroup>