Hi,
The Alternative URL feature is NOT a replacement, it's used as an alternative way. Main purpose is to have a short campaign url for instance.
If you want to use the alternative URL you could get it via API:
AlternativeUrlInfoProvider.GetAlternativeUrls()
.WhereEquals("AlternativeUrlDocumentID", yourpage.DocumentID)
.ToList();
Note: that this can contain no (if not set) or multiple alternatives (when multiple are set). This is culture specific as you are using the DocumentID and not the NodeID
Alse remember that this can cause SEO issues if you don't redirect to the main URL. So you also might want to add Canonical tags.
see kentico docs for creating Providing canonical URLs for pages