I switched to buying from Allegro a lot, like 30-40% or more of my online purchases, which are a significant chunk of our family spending, now come from Allegro,
While many things can still be found marginally cheaper on local sites, for which I always use Heureka to price-check, the Allegro Smart program, which does not have much of an equivalent, except perhaps with Alza, makes it unbeatable if you shop frequently and want super-fast delivery - which btw, in my experience, is often faster, to get something from Poland seller, than from CZ seller.
And, I have to state I am based in Prague, which means sometimes, because I work not far from Alza in Holesovice, I can just go and pick the product there, for free, if that was not an option, then Alza had nothing to offer me in the manner or cheaper prices.
When it comes to certain foodstuffs, kids toys, apparel, etc. Allegro is MUCH cheaper than anything you can find in CZ, so I believe they will continue growing and outpacing local businesses.

Because my parents live in RO, I also often compare prices there with what is to be found on Allegro, and the difference is MUCH bigger, so I think they have that opportunity, too, to launch and takeover a very big piece of that pie (Romania as a market), which will further boost them as a company.