> The Commission considers that low market shares are generally a good proxy for the absence of substantial market power. The Commission's experience suggests that dominance is not likely if the undertaking's market share is below 40 % in the relevant market. [1]
Historically the European Commission's antitrust cases against tech giants have largely been against companies with overwhelming market share:
European Commission vs Google: 90% market share in search [2]
European Commission vs Intel: 70% market share in x86 CPUs [3]
European Commission vs Qualcomm: 90% market share in LTE chipsets [4]
Apple's 25% is nowhere near the same level and I expect the Commission's investigation will conclude that they do not, in fact, have a monopoly in the EU.
[1] https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX%3A...
[2] https://ec.europa.eu/commission/presscorner/detail/en/IP_17_...
[3] https://ec.europa.eu/commission/presscorner/detail/en/IP_09_...
[4] https://ec.europa.eu/commission/presscorner/detail/en/IP_18_...
Generally in Europe the market share threshold for monopoly investigations is 25% - Apple's market-share for mobile app sales is well above that threshold.