Even buying a physical commodity and selling the direct analog futures contract is not technically an arbitrage.
Okay, but what about a futures arb against the underlying for a non-physical instrument (or a non-delivered instrument?) For example, BTC June futures vs underlying. Trade is long BTC, short futs. There might be some pain if the price goes up, but all you need is capital to cover the spread and you're fine.
Here are the numbers right now. That's an absolute return of 9% over 3.5 months. Annualized is a mouthwatering 36%. Whenever crypto is hot, those returns are there for anyone with enough capital to claim them. I would say that's an arbitrage, but I'm interested in your opinion. Thoughts?