Well, considering that the EF mount wasn't closed to those manufacturers, Canon has previously shown willingness to let other manufacturers engage. I would think the benefit changes over time. Today, there is benefit to Canon to keep it closed because having another manufacturer release something Canon plans to release eventually will give those other manufacturers a competitive advantage and impact Canon's long-term revenue. For instance, if Sigma bolts on an RF mount to their 14mm 1.8 before Canon releases a 14mm in RF, yes, people who may have bought the RF version may buy the Sigma instead because it's available today and Canon's version doesn't yet exist. Competition here is impacted by being first to market.
Once Canon has rounded out their offerings on RF, the benefit may change to make opening the mount more favourable. Competing with Canon wouldn't be about being first to market with a product, but rather finding niches where they can find some success because Canon hasn't filled the niche - whether by being a lens that is a low volume speciality product, or lower cost/quality option. If a secondary manufacturer wants to release a lens that Canon doesn't believe will be profitable enough to warrant manufacturing, it's a benefit to the system to have someone else do it - more lenses available, Canon wasn't planning on selling that product anyway, reduces some of the noise about the closed ecosystem, wins for everyone.