My mom, in her 70s, barely gets iMessage -- half the time I receive "text" messages from her as an email! Throwing in Whatsapp or Telegram would just confuse her.
My grandma in her late 80s managed to learn a non-iMessage messaging app just fine, and she’s the most active one in my family group. If grandmas all over the world can use something else, your mom can, too. I’m sure she wasn’t born with the ability to use iMessage.