People don't care about decentralisation, but one of selling points of the Fediverse may be pretty similar to that: interoperability.
IMHO, some separate but federated servers make sense for a newcomer more, than other. They are:

• Flagship Mastodon server (for the "official blessing"),

• Several very large Mastodon servers for niches - Infosec, FOSS and Gamedev communities already have got these (for best discovery of that niche communities and still good discovery of out-niche content - thanks to size),

• At least one national Mastodon server in your language - of course open to registration - Japanese, British, German, Italian, French, Dutch, Canadian, Australian, Chinese are the largest, but any language-specific will satisfy the user (for best discovery of content in their languages or from regions - helped by language barreer),

• Some largeish Mastodon instances with specific political leanings - anarchist, far-right and libertarian ones are the largest (for better-tailored moderation, and also for discovery),

• Flagship/largest instances of other Fediverse software, such as Akkoma, Lemmy, /kbin, Bookwyrm, Misskey, Calckey, Pixelfed (for features of their software, being their advantages over Mastodon)

• Some national instances of Lemmy, /kbin, Bookwyrm, Misskey, Calckey, Pixelfed in your language (combined language and feature reasons),

• Few Lemmy instances with specific political leanings - communist and far-right ones (combined moderation and feature reasons).

We do not have to talk about "decentralised" insert app name here network, but we can rather promote "Large Infosec-oriented microblog which interoperates with entire of rest of Mastodon", "German Reddit-alternative aggregator which interoperates with fedi", "A combined aggregator and microblog instance, compatible both with Lemmy and Mastodon", "Anarchist social network with access to much of fedi", "Mastodon-compatible microblog with fancy formatting", or "Instagram-equivalent with access to Mastodon but larger photo limits per post, and filtres", etc.

This interoperability means, that any new and sufficiently promoted (or simply receiving another Great Migration) server can claim over 1 million Mastodon&co users as the potential audience.

If you were already present on both Reddit and e.g. Facebook, and recent events forced you to switch from Reddit to /kbin or Lemmy, then you started to be on the Fediverse. And any your friend from Facebook started to be able to contact with you on the Fediverse, should they switch.
If you were present previously only on Facebook and Twitter, and you friend was only on Facebook and Reddit, then, after Twitter and Reddit Migrations, both of you began to be able to talk to each other, without the need of Facebook!
Every great migration to the Fediverse sows the seed. Sooner or earlier, all your friends will be present on fedi, and the network effects will begin to work to the advantage of fedi, not against it.