Unless you’re using mixers then it is relatively trivial to follow your BTC around the blockchain. That is the very point of it.
The issue of provenance is an issue regardless of the type of funds.
In most jurisdictions the burden of proof is civil, so more likely than not.
Which is a more complicated scenario then originally claimed, which was "I bought a $100 of BTC in cash many years ago and held onto it since then". If you cashed out that BTC via an exchange for example, then there will be bank transaction records showing you did but also welcome to "you were likely due taxes on your gains in the interim because it's no longer $100 of BTC - you took possession of much more in cash and later bought BTC again". Which is to say, you were ongoingly interacting with the money at decreasing intervals from the present.
The original claim is easy to prove: it'll be in a wallet which was held for that amount of time. The bank isn't obligated, unless you turn up covered in blood, to prove that you didn't just beat a guy with a hammer till he gives you the password - because you'll go down for that crime by other means.
But very few people engaging in ongoing criminal transactions are going to have a supply of aged BTC accounts to use in trade for goods and resources, and certainly you'll trigger red flags if you keep turning up with a brand new "held it for years" account of $50,000 each week. I mean we know this: because it makes headlines when untouched BTC accounts start moving.
I genuinely don't understand why mixing plus swapping into monero doesn't make sense.
Simply swap into monero and then send it to anybody else.. Maybe lets say you have 100$ in btc, you convert it to 100$ in monero and just spend 10$ 10 times in a span of a week and I am sure that nobody can keep track of it.
Suppose you deposited btc into an exchange, traded it back and forth for a while, and then withdrew. The withdrawn coins come from a communal exchange wallet, so the chain of evidence on the blockchain is lost.
Sure, you can trace the full transaction history of any BTC through the blockchain... but that doesn't prove anything. At minimum, you need all the private keys involved (e.g. not possible if you used an exchange or deposited/withdrew from some online service or lost/deleted an old wallet). Even if you had the full list of private keys involved, going back to 2012, what proves that they were really yours and, like another commenter pointed out, that you didn't just acquire them yesterday?