For example let's look into the price of Bitcoin on Bitcoin pizza day
2010: $0.0041
2011: $6.47
2012: $5.07
2013: $120.96
2014: $508.41
2015: $238.17
2016: $445.23
2017: $2,136.02
2018: $8,465.99
2019: $7,623.51
2020: $9,205.40
So, if a vendor accepts Bitcoin he/she will benefit and not incurr any loss.
This is a really bad of thinking about it, business owners aren't long-term investors, they are mostly focused on short term. Just look how covid-19 impacted small business, so many of them were forced to close because they can't afford a month or two of rent when they have no income. The same can easily happen with Bitcoin - imagine a business that mostly accepts Bitcoin and doesn't convert it, and one month Bitcoin crashes by 50% - now they are at a loss and will have to take loans or get closed, but it's still a risk because there's no guarantee that Bitcoin won't fall further.
If a shop wants to accept BTC, they need to be smart about it, have a solid strategy for converting to fiat, prepare for it legally, maintain proper security, etc.