Not OP but in a similar situation.

The main lock-ins for WhatsApp with my friends/family/colleagues are:

1. Group chats. SMS group chat doesn't exist (or it's next to unknown) in Australia.

2. Sharing images and videos. SMS destroys images/videos/gifs (if they even send).

3. International. Messaging friends/colleagues when they're overseas is easy.

4. Videochat (however, it's usually FaceTime with an older relative).

I attempted a shift to Telegram with a few close friends and family members. Eventually, we started to drop back to the "normal" comms route because our extended network was on WhatsApp/iMessage and juggling several methods was irritating (e.g you message a friend on Telegram and get no response -- they then message you later that day on WhatsApp -- it's irritating to move the conversation back to Telegram).

I don't use any of the private and popular messaging apps on my phone and do rely on SMS and phone calls to stay in touch. But there are limitations:

- SMS is not encrypted.

- SMS supports text only. MMS is not well supported, and often not free.

- SMS is sometimes not as "instant" as it can be delayed.

- Delivery reports and, read receipts are not user-friendly, and maybe unreliable, too.

- Group SMS support depends on your default SMS app.

RCS or Rich Communication Service on 4g and 5g looks to fix this, but support and compatibility between network is still lacking. Privacy laws also need a reevaluation as even cellular providers are looking to data harvesting to make more money and RCS may also lack encryption support.

To list a few benefits:

- e2e encryption

- many extra chat features (reactions, stickers, replies, polls, etc). It might seem unnecessary but imo they do genuinely increase functionality and ease of communication

- scalable to large groups (maybe sms is as well, I've never tried more than 3-4 people)

- don't need a phone, can message from a computer instead

- messages sync across multiple devices

- video calling for groups with some apps

Most phone companies on their lower tier plans make you pick 1-2 of free calls, texts or a data allowance. High end plans aren't nearly so popular because most people don't get high end phones and of the people that do, many buy direct from Apple so don't have a contract associated with it, instead just using a prepay or sim only plan. So it's really only the high end android phones which get bought on contract which is a much smaller market than iPhones or the actual big market segment here: €100-200 androids

Nobody picks free texts. This leaves 15c/message as a discouragement for using SMS.

For impromptu groups when you don't want to give your number?