I would have never started playing Splinterlands years ago, if I knew that it would require me to constantly pump money into new cards ...
Simple as that!
In my opinion it doesn't help the overall game economy at all to devalue old cards. If I felt my old cards would keep/increase(!) their value, I still would consider to buy new cards, as well. However, as I observe my old cards to lose value and use cases I would be just stupid to buy new cards again (which will be 'old' sooner or later, too).