Looking purely on graphic card, the only reason I can only imagine for losing performance is cooling. In five years, quite some dust can collect on heatsink. If you are otherwise happy with 1070 (at current market, you need to be), then I suggest you carefully disassemble graphic card, give a GPU cooler a good cleaning and apply new thermal paste -cost you nothing and you can only benefit. Saying that, I don't believe graphic card would "age" over the time.
Other issue could be.. anything: PSU doesn't perform as new anymore.. or Windows became bloated over the time -is VERY worth to make clean install before you disassemble graphic card.