I've seen many startups hit the end of the term of a convertible note before raising an equity round, and it has never once been a problem.
Maybe I'm being a bit pessimistic, but this really sounds to me like "US housing prices have never fallen by X%".
Investors who fund a startup using convertible debt are not hoping the debt will be repaid, like a regular lender is; they want it to convert to equity.
I'm sure that this is correct at the point that they provide the funding; I'm less certain about the situation 18 months later. Do you really think that no investor will ever change their mind over the course of 18 months and decide that they'd prefer to cut their losses rather than extend the term of an investment in a company which no longer seems quite so hot?