Policies with protectionist side effects (even if they're not marketed as such) have historically led to local businesses being less capable and less competitive over time. Whereby there is no need to compete or innovate as the business is insulated from genuine competition.
My assumption is that the EU believes this will lead to local businesses having the breathing space to grow to a critical mass where they could compete more robustly.
Looking back to historical examples we saw that businesses that benefitted from artificial protections were less competitive than ones that did not receive a benefit. We also saw that favoured businesses tended to be trapped inside the market where they receive those protections, i.e. they were optimised for those conditions. We see this more contemporarily with protected Russian and Chinese firms.
I am also curious if state-sponsored competitors will engineer a way around being labelled a gatekeeper. Such as by having a range of products with shared intellectual property spread across a number of legally discrete entities, effectively using a distributed form of anti-competitive practices.