The most obvious example of a technology that was initially great and went downhill (in my opinion) is Cryptocurrency.
When Bitcoin was first introduced to the world in 2008-2009, it was exciting and seemed very promising as it addressed some of the biggest problems with fiat currency and traditional payment methods:
However, things changed when the focus of crypto went from being a currency to an investment. I'm not against investing, especially long-term investing. However, crypto has now become a hotbed for scams, people launch meme coins only to abandon them (known as a rug pull).
Since the focus of crypto is now investment, custodial exchanges (e.g., Coinbase, Binance) have turned crypto into the thing it aimed to destroy. They know who you are (because of KYC requirements) and they have the power to ban your account and seize your funds. Since Monero (XMR) is a privacy coin, most exchanges do not accept it.
It should go without saying that I'm highly against using crypto for crime (e.g., money laundering), but whether your actions are justified or not, the fact your account can be seized at any time is proof that exchanges aren't any better than banks. In fact, I'd argue they're even worse since you're not protected by schemes like the UK's FSCS if you're a victim of cyber crime.
The Dark Web refers to anonymising networks on the internet that can only be accessed using special software. There are quite a few gateways to the dark web (e.g., Zeronet, I2P), but the most well-known is Tor (The Onion Router).
On the surface, Tor looks like a noble thing. It allows people to access the internet anonymously, bypass censorship, and access sites which aren't indexed by regular search engines (ending in .onion). So Tor is important in countries where the press is less free and where people can be imprisoned for saying certain things.
That said, just like with crypto, there are malicious actors on the dark web. People who use it to deal drugs, sell firearms, buy stolen credit cards etc. As such, there have been cases of relays being set up to try and deanonymise users.
Of course, a lot of malicious actors are caught due to poor OPSEC, but this action combined with the Tor project neglecting certain issues (e.g., weakness to BGP hijacking, security controls in Tor Browser) has lead users to question whether they can still trust Tor.
Torrenting hasn't exactly "fallen off" the way crypto and tor have, but there is a very common misconception that "torrenting = piracy, piracy = illegal, therefore torrenting = illegal".
However, this is a misconception. Torrenting itself is not illegal, it's simply a decentralised, peer-to-peer method of file sharing. It has some legitimate benefits such as reducing the load on a central server and having a backup if the site goes down (temporarily or permanently). Torrents can be and are used for legitimate reasons such as downloading Linux ISOs.
What is illegal is downloading copyrighted material. So if you download a movie, the crime isn't torrenting, the crime is copyright infringement. It wouldn't be any more legal if you did a direct download.