My rigs have been running for over 3 years and I'm just started to sell cards to refresh. If you have low electrical cost and don't buy over priced cards you'll do fine. My electrical cost is low and I bought my cards open box or refurbished before the craziness in 2017 and 2018.
I'm selling my 1070s and replacing with rx 5700s. I'll sell my 1070s for around 190-220 on ebay and buy refurbished 5700s XTs for around 339. After I sell a 1070 and pay taxes and fees I'm paying 150 for a rx 5700 xt.
I also hedge my earnings, sell short btc or eth in a down trend.
That is a good strategy, only you know if is worth or not, for me trading and buy and hold is better. The 5700 might be outhashed by the new nvidia gpu's, well, all depends of the prices the new gpu's will come too. Even if eth changes to progpow you can still mine with the 5700 and 5700 does double hashrate x 1070 on rvn.