A few reasons someone might want something purpose-built:
1. Video/audio messages. I wanted to record something for people, not just send text.
2. Multiple recipients with different messages. Gmail sends one notification to designated contacts.
3. Attachments that outlive your Google account. When your account goes inactive, your Drive files go with it.
4. The "who survives longer" question is real. My answer: one-time payments (no churn pressure), minimal infrastructure that can run unattended for years, and I'm exploring escrow/trust structures for long-term continuity. Google's killed plenty of products too (RIP Inbox, Reader, etc).
But honestly, if Gmail works for your needs—use it. The goal is that the message gets delivered, not that I get your money.