First of all, "lost" Bitcoins are different from "stolen" Bitcoins.
Lost coins are those which can't be spent, as the private key has been lost. You can easily trace these coins, as they can't be moved and will remain in the original wallet. By checking the Blockchain, you can find the details.
Stolen Bitcoins can be difficult to trace if the thief uses a Bitcoin mixer. Even then, they can be traced with some effort. Back in 2013, Tomáš Jiřikovský and Eva Bartošová stole a few millions worth of coins from the users of Sheep Marketplace (ironically, Jiřikovský was the owner of SMP). He used multiple mixers, but eventually he was caught after tracing the transactions (he pissed off a lot of powerful people. I don't know what happened to him after he was caught).
Exactly. There's a huge difference between lost and stolen Bitcoins. The main reason why coins end in such a state, is largely attributed towards irresponsibility by the user. An individual can simply choose to secure his/her private key against theft or loss, by following well-known practices within the crypto industry. Still though, if Bitcoins are lost or stolen one way or another, we can always resort to the Blockchain in order to keep track of the origin and destination of the same.
It's easy enough to track & trace a transparent Blockchain like Bitcoin, but nearly impossible in fully-obfuscated blockchain ledgers. With Monero and Grin, people who lose their coins won't be able to know where they went. The same thing happens with Bitcoin if the you or the hacker used a mixer/tumbler, or another privacy solution to obfuscate transactions. Knowing these issues, we may not be able to trace every single coin at all.
Despite this, I believe that the more coins are lost, the better it'll be for the scarcity of Bitcoin. Such scarcity will greatly benefit Bitcoin, as prices become higher over time. There may be even lesser coins in circulation than we've ever thought. Perhaps, someday as Blockchain analytic tools improve, it'll be much easier to track how much coins are currently circulating, as well as, lost/stolen ones no matter if mixers/tumblers are used. After all, there's no such thing as 100% anonymity. Somehow, transactions will be discovered by any flaw existent within currently used privacy solutions on the Bitcoin network.
Nonetheless, keeping your public keys secure would allow you to at least trace where your coins were sent across the Blockchain. You could lose your private key, but losing both keypairs (public and private) will only make matters worse as you won't be able to know where your coins are located within the Bitcoin blockchain, neither gain access to them. Which is why, it's important to take good responsibility of your hard-earned coins to prevent any of the aforementioned situations from happening soon. Just my opinion