You should be able to log in with your sims3.com account. For the average Sim player, it's just another "EA Download Manager" with a different name and coat of paint. You can still update your game manually to avoid the Launcher/Origin, but for those who don't do that, they will likely never notice the change.
You'll possibly see EA take their games down from Steam over the next year or two but not anytime soon(Crysis 2 was recently taken off Steam with EA responding that a contract was broken. Whatever that entailed). I don't QUITE get why they'd take this stuff down regardless(why lose more revenue options?) but then again, Nintendo/MS/Sony all compete for exclusives and they make some good money(even if putting Mario on
all consoles would make more money for Nintendo-for example), and I'd say EA is arguably far more known than Valve/Steam to not care what Steam fans think about their recent Origin program. I think you're going to see a huge digital distribution(DD) market heat up in the next 5 years and EA wants a piece of it.
I personally love Steam, and while I don't buy Sims off of Steam like all my other PC games (I haven't bought a physical copy of games for a few years now, but Sims I still support it!), I've seen some awesome sales with Sims 3 stuff packs and expansions on Steam ($10 stuff packs, $50 for Sims 3+1 expansion, etc, just to give you an example of what I've seen). It's too bad Pescado/AM won't support Sims 3 for Steam, but he's now kind of forced into supporting Sims 3 on Origin, unless you download your patches manually. Maybe that's a baseless argument(he'd probably tell you you're stupid for using the launcher and not downloading patches manually), so he'll probably say the same for Origin that he says for Steam: Screw you both!
I don't personally find EA as a company who the consumer benefits from, where as Steam has saved me SO much money with all their awesome sales. $10-$20 for blockbuster games, $2-5 for indy developers, etc.
As a consumer, I have massively benefited from Valve's business model and have saved a ton of money. In EA's eyes, it's just profit going out the window for them and their shareholders. *(large opinion alert)* The Sims store is a massive rip off and a slap in the face of millions of people who've purchased hundreds of dollars of the Sims franchise over the years, and overall have given EA
millions of dollars in profit. If you remember the Sims 3 launch, you heard quite a fuss about EA wanting to launch Sims 3 ASAP for their shareholders.
That's an insane tell of who they care about more. If they cared about their customers, a new world once a year for free isn't a big deal
at all. Offering a set of furniture for free every month or quarter/season is also-not a big deal from a multi-million dollar company. They can easily afford it without taking any hit what-so-ever. (Trickle down economics, hello!? Teeheehee) But
none of that sounds good to the people that invest in their company and want everything profit based, and EA cares a lot about their shareholders. Digital distribution is nearly pure profit(it's not cheap to manufacture and print games for retail. DD is just supplying bandwidth costs which is significantly cheaper than retail) and they haven't given as much to the Sims community as they did in the past when DD wasn't as common/marketable or,
sellable. I do have to give the Sims team awesome props for including content in the patches. That's pretty impressive that they have included some meaty expansion content for base game players.
But that is my sole reason for not supporting Origin. EA is a company out to make profit to cater to their shareholders, while caring absolutely nothing about the people that feed them that profit. Look...I may hate on EA, but I have much admiration for the Sims team. I fully believe they are out to make as best the game/expansion they can for Sim players, but are severely limited by upper level EA management. I hope one day we can detach the Sims away from EA and recreate Maxis(Glass/Steagall anyone?

Come on you history buffs...). They make far more than enough money to support a Sims/Maxis company, but I think EA now has a legal grip on the franchise.
Woh...that was a long post.