A key factor is the % of one’s wealth which is in Bitcoin. If the % is low, it makes sense (from a bullish on Bitcoin perspective) to spend out of fiat first. If and when the % nears 100% then spending one’s Bitcoin will become more of a necessity (assuming one doesn’t use the Bitcoin as collateral for a fiat loan).
Both the right ideas, the timing and the major flows of capital need to intersect for a paradigm shift to occur.