That's false. Encrypted messages aren't searchable by third-parties. End-to-end encrypted messages are searchable by only the communicating parties regardless of where they're stored.
There's also the issue of mirroring by third-parties, but then we're talking about data in transit versus at rest, your backup methods, and so forth. (And I don't have time to get into that this morning.)
Telegram is used a lot in developing countries which are using phones made 3-7 years ago. Or phones that are made recently but with old chips and tech. Maintaining a search index across literally millions or billions (some groups are really really big) messages isn't something your phone can reasonably do.
I do wish telegram was e2ee for IMs, but for groups and super groups and channels I don't particularly care.
Of course there are people who are much more popular than me, so it might be possible and I am not aware of it. :)
https://t.me/durovschat Has 9500 members
https://t.me/swhkdemocracy Has 6000 members
https://t.me/linux_group Has 6000 members
https://t.me/PublicTestGroup Has 18000 members
There is no limit to joining these chats. These design decisions Telegram has picked has allowed the app to be quasi social media, rather than just IM. You can't index these and maintain chat history locally. You'd need way more bandwidth and energy and computational power than the average cellphone has.