I am not a military expert, so that's certainly a reason why I can't follow everything in this article. The Bruegel analysis the Economist mentions, however, says:

From a macroeconomic perspective, the numbers are small enough for Europe to replace the US fully. Since February 2022, US military support to Ukraine has amounted to €64 billion, while Europe, including the United Kingdom, sent €62 billion. In 2024, US military support amounted to €20 billion out of a total of €42 billion. To replace the US, the EU would thus have to spend only another 0.12 percent of its GDP – a feasible amount [...]

A significantly more challenging scenario for Europe would be an unlikely peace deal accepted by Ukraine. In such a scenario, Russia is likely to continue its military build-up, creating a formidable military challenge to all of the EU in a very short period, given current Russian production. The EU and allies including the UK and Norway would need to accelerate their military build-ups immediately and massively [...]

It also says:

A Russian attack on a European Union country is thus conceivable. Assessments by NATO, Germany, Poland, Denmark and the Baltic states put Russia as ready to attack within three to ten years 4 . It could be sooner [...]

Europe’s first priority is to continue supporting Ukraine – Ukraine’s experienced military is currently the most effective deterrent against a Russian attack on the EU. If Ukraine decides that a US-Russian deal to end the war is unacceptable – because Putin’s peace guarantees are not credible, for example – Europe is capable of providing additional weapons to Ukraine to ensure its fighting capacities remain as they are currently. Ukraine and the EU rely on some critical US strategic enablers, including intelligence and satellite communications. These are difficult to replace in the short term but there are substitutes if necessary [...]

Rapidly generating such increases [in military equipment and production] requires an extraordinary effort, though experience [in Eruope] shows market economies can do it [...]

Bruegel says -unsurprisingly- that Europe must significantly increase its defense spending, and also makes suggestions how this could be done best (amongst others, by replacing the US military-industrial base). Overall it provides a different picture than the Economist imho.