Well, I can see how you are in a much more vulnerable place when you haven't explored poly yet, and people will maybe even try and stop you... However it seems to me you avoid a few things that way:
- Having the whole "I can't believe you lied me for X long" thing from everyone, on top of all the rest, when you do come out
- Hiding someone you love, and therefore not giving them the same level of respect you've always given your first partner by acknowledging them (unless you were hiding them as well at first?)
- If someone finds out before you can come out, have them misunderstand the situation, draw their own conclusion and never come to you, but spread it to other people, in effect causing you to suddenly lose a bunch of people who were important in your life without ever being told why or warned before being cut off (I'm saying that because it personally happened to me)
- having your polyamory be associated with the person you're with when you come out. Similar to how homosexuality can become your bf's or gf's "fault", as in they seduced you and made you believe you're gay, or something. In this case, that person gave you crazy ideas and is manipulated you, and is obviously unsafe to be around, etc
I realise the risks that go with coming out are high. And it can seem more tempting to be able to say "I'm polyamorous, and yet it can last, it's been X years", but during these X years and at the time you come out, you have to go through things I would personally rather avoid.