Williams is rule 5 eligible in December 2023. Conine in December 2021. Robertson (if he's the one going for Walker) in December 2022. I don't think it's fair to say cleansing the roster of potential Rule 5 clutter was a consideration in any of these moves.
Whether Williams for Stripling was worth doing is another story. I know nothing about Williams and he's a few years away. Stripling comes with 2 years and a month of control, and his numbers as a SP in 2018-19 were very good (176.1 IP, 3.47 ERA, 3.68 FIP, 3.08 xFIP). Given where the Jays are in their competitive cycle, this was likely a fair trade if they envision serious contention in 2021-22, which I'm guessing they do, and if they think Stripling will pitch to his 2018-19 form moving forward (which I'm sure they do otherwise they don't make this move). The only reason this might look bad in hindsight is because the Dodgers player development is excellent, but as mentioned if Williams ends up being even as good as Stripling it would be an accomplishment. Pitching prospects suck.