I think that Anna's pregnancy afforded the opportunity to explore the storyline, rather than necessitated the change in storyline.
They don't need a real life pregnancy to write a fictional one. If they'd wanted to write a Danny/Lindsay pregnancy they could have at any time--Anna didn't need to get pregnant for the writers to script a pregnancy for the characters.
On the flipside, Anna getting pregnant left them with the decision to try to hide her pregnancy again--which they obviously didn't do very well in season three--or write it in.
Given that Rikki was coming back for multiple episodes, it's hard not to wonder if she was supposed to come back carrying Danny's child--especially since her return was scrapped as soon as the decision was made to write Lindsay's pregnancy in.
If the Lindsay pregnancy was really where TPTB were headed, Anna's real life pregnancy wouldn't have caused such a major change of course. But it wasn't--it was a storyline dictated by a real life circumstance. They did the best with what they were handed.
I've seen several shows that have not included a real-life pregnancy and, because we all know it's fiction, we all laugh at the outlandish ways they disguise the pregnancy and enjoy the show anyway. I would rather they give marriage a try, and if it doesn't work then they can both say they gave it their all and move forward from there.
Maybe if the couple had worked in the first place, but they didn't. They had what, one or two happy episodes? There was no basis of a stable relationship there with which to build to a marriage. No foundation, no loving base. Lindsay didn't even want to tell Danny she was pregnant!
What I do see is that since they introduced the baby Messer storyline that the couple has grown together and seems to be moving in the right direction. If anything the storyline has given a sense of resolution to these two that have been on and off for three years now. We never saw enough to really say whether they had a relationship or not. Now thay have one and seem to be working together instead of against each other. That is progress in my book.
I think it's progress, but it's forced progress, dictated by the storyline rather than by anything organic between the characters. I believe that Danny and Lindsay are happy about the baby, but they're still not a team in the way married couples are supposed to be--he lies to her about what he did in "Point of No Return" and is focusing on picking out baby names alone. She texts him with the sex of the baby. That's not a together couple. The marriage, at this point, was a big mistake.