First off, I'm not sure what you mean by completed.  Finished a plotline?  Reached a permanent conclusion?  ...had everyuone die?

I've done several of the above, but you have to realize that many rps and tpers, even if they start with a particular plot, don't actually /have/ a definite endpoint.  Even if the overarching plot is finished, people often want to keep going, and so the rp doesn't always ens.  ThjThjis isn't a videogame with a set win condition and endpoint.  Rp is most often a sandbox and maybe there's a big plot to complete, but you can still do all sorts of things in the world.

In terms of commitment issues, I see them all the time.  In most cases I am very committed, and I get invested very quickly.  There are people I've not heard from in years, but if they suddenly turned up again I'd continue an old rp or start a new one with them in an instant.

Sometimes taking a break can keep people interested, and sometimes it's just a prelude to final rp death.

I also happen to be a weird combination of patient, impatient, and a generally fast poster (with fast partners and sufficient inspiration).  No, most people are not going to post daily, not on forums. Oddly enough I used to find people that'd post multiple times a day, but for the most part I don't really encounter people like that on this site.  I think the constant pressure on people who like in depth lore to post super long things every time is a contributing factor, personally.  I treasure the world builders who will let rapid action and dialogue exchanges drop below a paragraph at times, if there's really nothing to say.  A willingness to move time forward also helps.

There's an issue wit people flaking or taking on more than they can chew, and you definitely need patience, but I've found dedicated partners and concluded several rps.  The most often completed ate ones that are tabletop -- for some reason, following module seems to generate a definite endpoint.  In a similar vein, having a definite goal and reading that goal, whatever it may be, creates a solid ending.  Then, if you want more of those characters, you start a /new/ rp that is a sequel to the previous.  Thread rps don't seem to have that sort of finish very often.  I can only think of a couple, offhand.

I've had a few 1x1 rps progress through multiple years of the chars' lives, and we mutually decided to write a conclusion and let the chars go off on their own without author oversight.  There's even been a couple thread rps with small groups that have reached a conclusion, and by mutual agreement been written to a similar end.  In most casees for forum group rps, this just does not happen.  It keeps going until it dies, even if it's for years -- anI I have o deed had some go for multiple years.  I've had two sequential FATE campaigns set in fantastical ww1 era finish, and we're on our third, with the onset of the war.  The DM for that is also one of the leads of a chat group that I've been in for like...seven, eight years now, and it's been around even longer.  This group is unusual, especially now, for several reasons.  Firstly, while we're down to 3x weekly what with jobs and all, we used to play real-time 7 days a week -- and back then, ooc time and IRL days lined up (they don't anymore, obviously).  Secondly, we started with medium lore and now have deep lore, but the game itself is actually script style, partially to facilitate that rapid-fire flurry of posting.  We do this in a chat room and I've seen upwards of ten posts in a minute, just to give a ballpark estimate.  This group has had people join and leave over the years, but since I've joined we've concluded four different campaigns, and we're nearing the end of the fifth.  Wait no, make that five done, almost six -- I forgot at one point we had two running parallel with different characters.  Each of these campaigns had a bunch of different smaller arcs, with the campaign being more a matter of setting and characters.

In the end, you have to be committed, have the persistence to keep looking until you find the right people, and then just get lucky.