To the IRS, from a taxation standpoint, Bitcoin isn't treated any differently than any other tangible asset. Whether real estate, art work, gold bullion, FOREX, or practically anything else the IRS position is more or less "bought, sold, gains and anything else whatever it's worth in USD, we'll tax it with current law". Sure there are very specific loopholes (for real estate development, famously) but those are an entirely different topic.
I'm not the most sophisticated in tax avoision strategies but other than straight up illegal evasion I don't see how Bitcoin changes the picture at all. Oh yeah and as far as Bitcoin having it all on a public ledger that just makes their job easier. Don't even start with tumblers or whatever else... Taxing authorities have been tracking down income since the days of Rome. They know what they're doing. I believe the current stats for IRS enforcement measures are they return $4 for every $1 spent on investigation, recovery, and prosecution. When it comes to a government agency they likely provide the best ROI (and I'm betting by a large margin).
Simply put, if you're evading taxes with bitcoin (or virtually any other method) and they decide to even take a peek at you you'll likely get caught and upgrade your charges to criminal tax evasion. While there is some cherry picking of cases to game this statistic last I checked the Government has a 97% conviction rate in Federal cases. Once again, unless you're "too big to fail" if they investigate evasion you're going to prison.
As for loans, you've ALWAYS been able to take out loans against other assets - it's a common leverage tactic and some would argue it's stupid not to. Loans against secured (real) assets typically come with comically low interest rates because there's less risk to the lender (the asset can always be seized). Think mortgage vs credit card.
I haven't looked at this crypto lending yet but I would imagine that as part of the loan either the private keys of the wallet or the wallet itself would need to be placed in escrow (or similar). If you default on your loan the escrow company just moves it to your lender (much like a bank, storage facility, etc receiving a lein or judgement against you would). It's just that in the real world for physical assets the bank has to pay for the police to show up at your house and kick you out or seize the asset at gunpoint. Once again, Bitcoin just makes their job easier.
When comparing interest rates to potential asset appreciation it's borderline stupid to have liquid capital sunk in to an asset you can loan against. Sure you're kind of rolling the dice in some regard but if over the term of the loan the asset appreciates more than what you paid in interest you made money from the loan. Trump famously got into trouble with this in the 80s/90s with some EXTREMELY risky leverage strategies.
But at level, as was said then and now: "When you owe $100,000 to the bank you have a problem. When you owe $100,000,000 to the bank they have a problem."
EDIT: When talking about escrow I completely forgot about smart contracts and the like. Either way it's quite possibly the easiest collateral to recover.