The more bitcoin is traded, the more miners it'll continuously need to keep the blockchain synced.
It doesn't work that way. While miners are free to store as much history as they want, most use SPV methods which only verify current history. It doesn't "create more work", nor is that the reason why hashrates (work) increases/decreases. Even if someone does keep a localized copy of the blockchain-the initial download is large, but cross-referencing is not a strain on the network nor the work required.
Then transactions will happen slower...
This is also misleading. The difficulty algorithm adjusts based on mining power available. If miners stop mining, the difficulty is decreased, to keep a 10 minute block . Though the retarget interval is about 2 weeks, something very serious need to happen to disrupt it. Even if that were to happen, there are mitigation options available to pursue if the need were to arise (forking).
The idea that bitcoin is "bad for the environment" is short-sighted and misses a few points. Though it doesn't serve as many people as the banking industry does, yet, it does use far, far less energy. Also, about 60% of miners use hydro-power. This is a supply/demand issue with electricity. If it is affordable to use electricity, electricity will be used. It is literally on the same competitive playing field as anything else that uses electricity. Blaming Bitcoin is a diversion from the issues surrounding the availability, sale, and use of energy itself--not how "morally valuable" bitcoin's use of it is.
Another thing people seem to miss, is that it doesn't matter how many miners are active, bitcoin will still function the same (albeit less secured with lower hashrates).
A rather large thing that seems to be missing from the conversation is the evolution of and the potential of bitcoin. Bitcoin has implemented a second layer protocol that allows hundreds of thousands of transactions to be embedded in a single "1 of 7 transactions per second" via a "settlement" procedure. It is estimated that it will be able to handle upwards to 1 trillion transactions a day, or over 11 million transactions a second --even if I was the only person actively mining bitcoin. Of course the lightning network (the name of the second layer) relies on people taking advantage of it to have their own form of repeaters/forwarders, but they are not at all energy consumptive. 1 raspberry pi could handle several businesses' worth of financial transactions with room to spare.
This is not, at all, trading advice, or encouragement to buy bitcoin. As for privacy concerns, it's about as private as you want to keep it. Selling it for fiat or trading it on some exchange that requires your government ID/compliance is going to expose whatever you are using with them. Privacy is attainable, but moot if you are already willing to dox yourself.
Bitcoin is capable of reaching lows lower than what we now call low, but it will certainly never be worth nothing-even if one person is willing to use it.