Unless, of course, you use places like Coinbase that pool transactions or bypass the network altogether, but then... what's the point of cryptocurrency?
For example, you buy 0.1 BTC using euros, you then pay a merchant asking for 0.1 BTC, which he will withdraw as dollars. Noticed that neither the buyer nor the seller manipulate bitcoin. No address, no transaction on the blockchain. It shows bitcoin on your account but in reality, it was a euro to dollar transaction.
Most likely you will be able to have a bitcoin account too, but again, dollars in, dollars out. If bitcoins raises in value, you get more dollars, if it crashes, you get less, but you never get actual bitcoin.
Edit: And BTW that's the reason why most people are using cryptocurrency for, as an index like gold. Besides criminals, few people use it to make payments.
There’s every reason to think in the longer term there will be some cryptocurrencies with higher costs that behave more like a custodial store of value like gold, some that behave like a cheap medium of exchange, and some that have other purposes like hedging against fiat currency or operating some smart contract based app / game / service.
The valuation model would be different for each.
You have to pay taxes on that gain.
There is also the problem that people are unlikely to spend a deflationary currency if they can avoid it.
Nobody forces merchants to stick to requiring block confirmations for e.g. Bitcoin transactions.
They just do it because that's the way it always has been.
Instead of that, for low price purchases, they could also monitor the network for how well-spread a transaction is and once it is sufficiently propagated assume that a double-spend won't be possible anymore because miners will refuse to accept a transaction which conflicts with the one they saw much earlier.
And "well-spread" would be typically the case within seconds because transactions are flooded to all network nodes as they appear.
At the very least there's no guarantee of that. If you send a conflicting transaction with a higher fee, a miner should (rationally) discard the older one. And last I checked (Ethereum, a few years ago), that was indeed what they were doing.
If miners did change their Bitcoin nodes to facilitate double spending even with RBF disabled, that would erode trust in Bitcoin and thereby de-valuate it, which is probably not in the interest of miners. They'd damage the thing they're heavily invested in.
Description of the "replace by fee" mechanism you're mentioning: https://en.bitcoin.it/wiki/Replace_by_fee
This page mentions that it is a "node policy", and from your comment I think we both agree that nothing in the protocol (which roughly guarantees that Bitcoin is fine as long as 51% of the hashing power belongs to benevolent actors) ensures these rules are in place.
Now, you're saying there would be some reputational damage which leads to devaluation which is bad for Bitcoin, so bad for miners, so they wouldn't do that.
This will not happen until such payments represent a big share of Bitcoin transactions. In particular, if a few merchants were to start accepting transactions that are not mined and just propagated, they could easily be "victims" (again, it's economically rational for miners) of double spending and there would be no impact on the valuation of Bitcoin.
Those take much longer than 6 hours to "confirm", no? The sender can chargeback these transactions months after the money shows up in your account.
You don't even need to wait for confirmations unless the sum is huge. It's quite safe to accept most 0conf transactions.
Layer-2 does alleviate some of the issues though, and all those talking about Lightning Network should look into Raiden.