I have had the benefit of lots of time to think about this. I was in an MC firm and left for reasons you will see below.

I speak for what i understood of the MC firm i was from, and the time i spent there. Silver circle firms, local firms, i do not speak for those, their requirements are easier to meet from what i have heard.

(1) You need a big book of business, to make partner. It is not a small number like S$500k annual revenue. Think several times that. This is a big book, and you don't make it from a one-time big deal, those count only to a small extent.
(2) Most locals are not so enterprising, they dont think about growing their book at a young PQE (think, 3 to 5 PQE). Most start when it's too late, think 10 PQE! That is way too late and you become expensive for the firm
(3) When the firm pushes you to try for partnership, obviously you do not have a book of business. And it's very, very challenging. Numerous Singaporeans fall at this. Numerous Hong Kongers fall at this. Numerous relatively senior lawyers in all sorts of Asian jurisdictions fall at this.
(4) For those who started early, good for them. I was lucky enough to be encouraged to start early, and I did. In the next 5 to 7 years, i can imagine having built a big book by then. Work comes when you are known for what you do and give good service. People recommend you! That's the best part.
(5) Yes, partnership is possible, and i knew a few others my age or several years older, from other offices in the network, whom i felt were in the same boat - junior, but when you hit senior associate, you know you will have a decent book of business.
(6) However, I left, and some of those individuals i mention above, left.
(7) Why? Because those of us who have the balls and the gumption to think we can make partner in an MC firm, contrary to what all our local friends say, or laugh at us for thinking so, or dismiss us - we realize there are some pretty good jobs out there, outside of law!
(8) With the networking skills we picked up, we found a lot more outside. Big risk, but potentially bigger reward.

So that's it in a nutshell. Looking back i wonder if i should've stayed. I still enjoy chasing work, chasing clients and winning them over, getting to know new clients. But i also enjoy doing other things now, new businesses, no longer a slave to multiple clients (yes we all have clients wherever we work, but it's very different as a legal services provider). I even get less rest now, but it's so refreshing to not check emails for an hour (i'm still checking more than i should, but that's my personal problem).

When i was in the firm i would check every 5 mins (yes immediately after i shower). Why? Because they were MY clients, and i had to take good care of them. And it is this same behaviour which helps you build a book!