People mail, text, or call me. Those who use WhatsApp know this and have the decency to take that into consideration.
I'll gladly deal with any downsides if that means maintaining the principled standpoint that being a member of society doesn't require owning either an Android or IOS smartphone, being forced to run someone's black-box proprietary software, and agreeing with some foreign mega-corporation's dubious terms of agreement. It's harmful for democratic society as a whole to engender such a dependency.
Besides, if the experience of friends and colleagues is anything to go by, opting in to Whatsapp means you get its downsides too: that one group member who keeps spamming everyone with noise, the constant demand for attention, the gossipy, sometimes down-right stalkerish neighbourhood watch groups (glad my neighbourhood hasn't hung up those sycophantic Whatsapp street signs), not to mention Facebook looking over your shoulder at the metadata level.