Abraham and Jacob polygamists??

At no stage does it prohibit polygamy anywhere in the Tanakh (what non-Jews would call the Old testament). Polygamy was allowed, and practiced, in Judaism until the 10th century. At that point Rabbeinu Gershom published a ban that prohibited polygamy, and some other practices such as yibum, in order to protect women. This ruling of Rabbeinu Gershom is accepted as binding by all Ashkenazi Jews and, in the modern era, by the majority of Sephardia and MIzrachi Jews. there are some Jewish communities where it is not accepted and under Jewish law polygamy would still be allowed (but due to civil law it is not followed). In a very few instances, where there are Jewish communities in countries where polygamy is common (such as in many Islamic states- polygamy is still practiced.

Note: While allowed, polygamy was never seen as a positve act but rather a negative one. The Hebrew term for a co-wife is "Tsarah" which means "One against whom I strive". It is recognised that having more than one wife is a recipe for strife in the home and is heavily discouraged. This is one of the reasons that the ban of Rabbeinu Gershom has been so widely accepted- it is seen as a formalisation of a preferred state and a banning of an undesireable state.

Having more than one wife was hardly unusual in the Middle East. And having a concubine, which is a step down in legal status, was hardly unusual. IN the case of the founders of Judaism, only Avraham and Yaakov had those arrangements because they were permitted/required by G-d because the family arrangements of the Patriarchs would establish the fundamental process of the future of the Jewish People. Lot, Avraham's nephew and his character defects had to be banished from the Jewish People, Ishmael and His character defects, Esav and his character defects, the various character defects of the 12 sons of Yaakov, all had to be banished from the Jewish People. The multiple wives were part of that process.

Nope, they just realized it's better to have only one wife.

Men who want more than one wife just want to add variety in their life to prevent boredom with one woman. Polygamists defend their position most often by saying it prevents divorce and promotes family stability.

Sure...

Nope if you read in the Old Testament there were several instances when it was allowed to have more than one wife... other instances where it wasn't... its not an error in the Bible simply there was a need and then there wasn't a need... then a need... and so on...