Let's assume that someone buys "off the shelf" a Social Network (License) with all the core social network features needed, for a one off payment of 500$, and then a renewal fee of 250$ per each following year (of continued use). Then, the inhouse graphic designers and web developers modify the webpages (template) layout, the colours; and they adapt (change) the coding to their own needs. Over time additional plugins and other webpages are added and the original language becomes a smaller proportion than the originl 100% (of the COTS). When can a web development Team claim Copyrights "ownership" of their enlarged "coding" and stop paying the renewal fee to the Company from which they bought the COTS product? When is the line crossed between "It was yours on Day 1....but... it's mine now, as it looks to different from what I bought from you"? Do you have a ratio in mind, for example, old coding 30% new 70%? or how else could you possibly justify not paying whoever sold you that License and is chasing you for the yearly renewal?
When can a web development Team claim Copyrights "ownership" of their enlarged "coding"
Never. It's still their product. Changes to the product simply make it a "derivative work" under copyright law.
Until the changes and additions no longer rely on the original code (meaning it could work with a clean-room from-specs-only reimplementation by a group that has never worked with the original, xref the IBM PC clone BIOS), they can dictate licensing.
how else could you possibly justify not paying whoever sold you that License and is chasing you for the yearly renewal?
This would be completely foolish without consulting a lawyer to read every letter of the license, and not even then. Given the price range that you're talking about ($250 a year is small change), the cost of a good lawyer is going to cover a few years worth of licensing costs.