I don't think the number of shitcoin projects are declining because there's around 21K coins right now on CoinMarketCap, few years before it's only 2K. Growing up from 2K to 21k for few years is really a huge number difference.
I think Shitcoins hype start since 2017 which is already 5 years right now and there's nothing changed so far.
Although I really hate shitcoins, but I don't see any sign shitcoins era will going to end.
Until everyone realize decentralization and privacy is a big concern, shitcoins era will end ASAP.
I remember that in the year 2017 it was just the beginning of this time that cryptocurrency was becoming trending by releasing new projects via ICO, many were giving giveaways at this time. And just before the end of 2017, altcoins exploded and most of them were shitcoins I can say.
I also think that shitcoins will never disappear from the crypto industry in my opinion. Because, as long as there is money to be made here in the crypto space, the number of shitcoins in the market will surely continue to increase, so everyone should be careful and wise in choosing altcoins to buy.