The ending, with Ted leaving Marshall and Lily the old apartment, was sweet, but also confusing. I can understand Ted wanting to get out of that apartment, because it's so comfortable and familiar that I can see how he might feel that he's using it as a security blanket of sorts. And true, he knew that Marshall and Lily were miserable out in the suburbs. But did he really know that they wanted their old apartment back? I remember back in Season 1 that Lily didn't want to live there because it was a "boys' apartment", that she basically wanted a new place for her and Marshall to start their married life/family together. On the other hand, I think it was a great idea for the show to have Marshall and Lily move in there, because I think it'll be easier and more realistic for them to hang out at MacLaren's occasionally or for the gang to hang out with them if they're right upstairs with the baby, rather than in the suburbs or even a different part of the city.
I see what you mean. But Marshall and Lily have the opportunity to turn the apartment down. It seems like Ted moved out to shake things up a bit in his life, not because he thought they needed/wanted it more. They might sell the house, or let Lily's dad take it, and forget about it completely ... or they may hang onto it as a fallback option. Maybe they will keep it as a weekend/summer place, or maybe they'll rent it out to someone and be landlords. They very well might end up deciding that they don't want the apartment.
I don't think Season 1-era Lily ever wanted a suburban house. She seemed really happy in the city. Marshall, if anyone, was the one who wanted a white picket fence. She disliked being a married couple living with a roommate, not the apartment itself. Now the apartment is completely bare and she can set it up however she wishes, since it's only for her, Marshall and the baby.
I totally agree that moving M&L back to the city will give them more opportunities to hang out at the bar. The two of them schlepping a newborn out from Long Island wouldn't ring true to what most real life parents would do, and it'd be really crappy of either of them to say, "You take care of the baby, I'm taking a 46-minute train ride to go to a bar and spend time with my friends." That's at least three of four hours out of the house, and that's assuming they only have one or two quick beers - and who would take that long trip just for one or two quick beers? It's more realistic of them to bring the baby to the bar/restaurant downstairs to grab a hamburger for dinner, or for Marshall to say to Lily, "I'm just going to run downstairs and have a quick beer with Ted, then I'll stay with the baby while you do something to relax."