I think in that situation, it would still have been a better gamble than Renault who was going nowhere.
In that situation, it was an Honda, and not Brawn, actually, the team that made him the offer. In fact, Brawn, as Brawn GP, were as poor as they get, so they could not have afforded Alonso. The offer came from Honda, and it is, even today, anybody's guess whether Honda had stayed in F1 with Alonso on board, and what would have happened if Honda, and its engine, had remained in F1.
On the other hand, Alonso, on a Renault, (and with Singapore), had scored more points than anybody else, in the second half of
2008. That data may not be perfectly accurate (made it 5 points less than the second most scoring, if you want), but I am not trying to establish a second half championship, but debunking the myth that Renault was not going anywhere. In the second half of
2008, even getting Singapore out of the image, Renault was going places. It was in 2009 when they lost the plot (as McLaren and Ferrari did), and they haven't found it again since.
Pre 2009, Alonso said that he was only interested in getting one year long contract, and nobody offered him a contract for a single year. Brawn/Honda said that they wanted him long term, and RBR said that they were interested (IIRC, they said something along the lines "who would not be interested in having a driver of such standing here") but that they could not offer (yet) what Alonso was looking for.
Personally, I believe that Alonso was tied to Ferrari before the end of
2008, and he knew that he had to be available for 2010, either because he already got a seat at Ferrari for that season (and that seat would probably have been Massa's) or because it was highly probable that he could get one. In that situation, as he had an agreement with Renault, which had a very good second half of the championship, which allowed him to opt out of the agreement at the end of each season, he tested the waters with other teams, but he was really not so interested, and neither were the teams, considering the possibilities that he was going to Ferrari at the end of the season, no matter what.