Re: Not only startups lack proper testing
I can think of a couple of ways (from experience) how that could worsen the situation:
One: proper edge case testing for a complex function can easily take more than a single sprint. So testing will be made to fit the timeframe, instead of the other way round.
Two: there are systems where the "naive agile" way of bolting on user story after user story just is a recipe for disaster, because the end state was never properly defined and designed for. You will find out new edge cases for old functions as you go along, since you start to use and combine previously implemented functions in imaginative ways that take you out of the domains for which these functions were designed and tested. With added project pressure, hotfixes and workarounds will be implemented that will further constrain the future extensibility of the system for other customers. (just in case that needs to be added: e.g., systems that solve NP-hard LSCO problems)