The Court of Appeal decision in O’Brien-Glabb v. National Bank of Canada states the principle that the plaintiff bears the onus of establishing the inappropriateness of a proceeding as part of a discovery argument:
[13] We agree with the appellant that it was the respondent who bore the onus of leading evidence to establish on a balance of probabilities that a proceeding was not appropriate in 2010 (see: Miaskowski (Litigation guardian of) v. Persaud, 2015 ONCA 758(CanLII) at para 27; Fennell v. Deol, 2016 ONCA 249 (CanLII) at para16; and Galota v. Festival Hall Developments Ltd., 2016 ONCA 585 (CanLII) at para 15).
Even a vague familiarity with the operation of s. 5 of the Limitations Act means this principle is self-evident, but it’s nevertheless helpful to have it stated explicitly.