Why? EU can just tell an LLM to build an alternative app, they can just tell it to make it user friendly and make no mistakes. That's the primary use case of Trillions of dollars of investment in GPUs and electricity to power them.
JK(or am I?), a protest will be a boon for EU, which is growing Anti-American each and every day. The EU alternatives don't exist not because Europeans can't code but because EU market is open to US companies and there's no reason for duplicate effort as winner takes it all thanks to network effects. EU capital just invests in USA based companies that operate in EU. It's much easier, lower taxes lower worker protection standards etc. Also, US has much more capital to burn to corner the markets, they also just go ahead and buy anything European i.e. Skype. and not risk competition.
If the AI tech is real then Europeans get their apps no need for American companies anymore. If the tech isn't real then the chat app makers(in Europe that's all Meta, WhatsApp + Messenger. Apple is a no go due to low market share) lose half of their revenue to teach Europeans a lesson. Europeans them switch to Signal, Telegram(Popular all over the place) or Viber(this one popular in Eastern Europe).
If they all decide to boycott, there you have opportunity to hit a 400M market. Let's say Europeans can make AAA games but can't code a chat app, give visa to US developers that were laid off for the last many years to come over. Maybe it's politically unpopular? Maybe they are not the best? That's fine, EU is an open market - anyone who is interested in making billions can just make a chat app and take over the EU market. Indians, Chinese, Turks, Africans, Americans that earn less than billions of dollars. It's a huge opportunity. Instantly.
Meta made $38.4 billion USD revenue in EU last year. It's very unlikely that no one will be interested in taking that. US companies aren't doing charity to EU, it's always their second largest revenue stream after USA.