is't the features the whole point? why does people use WhatsApp, signal, Viber, telegram, messages or iMessage?
is it the user base or the features and User interface they have the deciding factor?
At one time features were the primary differentiators but as user expectations have evolved almost all messengers implement the same core features, albeit in different user interfaces. If WhatsApp were to become interoperable with iMessage, I'd ditch the WhatsApp client and use the iMessage client to reach my WhatsApp contacts mainly because there is no WhatApp client on my iPad and it's my preferred device. If Viber and WhatApp became interoperable I'd ditch the Viber App and all of their annoying ads and commercial sticker packs. It would probably put a serious dent in their monetization model if a lot of people did the same. Almost all of the monetization models of the different messaging services are implemented through the client apps. E.g. Advertising, tracking, subscriptions for group content access, stickers packs, etc... If I could have access to all these communities through a clean unencumbered and relatively private and tracking-free interface like iMessage, I would do it in a heartbeat. That is good for me as a user, but really bad for these services that want to stay in businesses.
if an upstart reaches a gatekeeper status, then the chance is they just replaced or became the new Facebook.
They would become the Facebook of 2013-2014 which was was stock-rich but operating on very slim profit margins. A Facebook mostly leveraging their stock value for acquisitions and revenue acquisition. Amazon was not profitable at all when they reached gatekeeper status. Neither was Snapchat based on user count and valuation, but I can't remember if they are considered an important enough market for the DMA to apply to them... kidding.
It mostly depends on the business model and the company's growth plan but its very easy to have a $30-100 valuation per active user during the growth state where a company could foreseeable reach the active user and global market cap threshold and yet be still hemorrhaging cash with an un-solidified and unproven business model now encumbered by a new set of regulations.
There are a lot companies like Telegram that exceed the EU active user criteria and are well over the halfway mark in global valuation, but have no revenue. Telegram isn't even planning to start booking revenue until next year. But an influx of users paired with irrationally exuberant investors could easily bring them over the gatekeeper line without them having to do anything.
and why would "new facebook" have to be sold to existing Gatekeepers? it can be sold to literary any other multi-billion dollar company that don't exist in the market. And why would you even want the existing players to buy up small competitors to be killed?
Even if Telegram doesn't make gateway status on its own, it's hard to imagine any company that would be in a position to buy them that wasn't already a gateway or wouldn't become a gateway soon after closing the deal.
While I guess it is possible that a random rich dude or company in an unregulated industry could buy one of these tech companies it rarely happens and when it does happen it's a fire sales that nets investors far less than the premium they would get from an in-industry merger.
The reality is that most businesses fail. Seasoned investors want to see they have as many opportunities to recoup something when that happens. The prevailing idea is you place and many reasonable bets as you can and mitigate as many risks as you can so the wins will outpace the losses. When you reduce the options for mitigating losses you change the dynamic of the investors' winning formula and they place less bets on things that have fewer exit strategies. Nobody wants to sell their company, but having that option is sometimes the difference between getting the startup capital you need or not.