(Western) polytheism failed for a multitude of reasons. In the context of Christianity (since I can best speak about that), it was a mixture of the hearts of rulers swayed (who would then order their entire population to be baptized, sometimes under penalty of death), the ease of having commonalities with trading partners, and plain conquering.
And of course, you can't dismiss the fact that there were many who were really convicted by the Gospel, or found it absolutely bright compared to a neverending cycle of giving stuff to apathetic or even malevolent gods in order to keep them from wrecking their crops.
For all those reasons, the strong gods of old were bested by a Jew on a stick.
There's also the whole saint stuff, where you pray to certain saints for protection as if they're minor deities.
That's not how that works, and they're not prayed to as "minor deities".
In the first place, you can "pray" to anyone because to "pray" is to request. In context, what occurs when "praying" to a venerated saint is plain communication to one, since they're still living-- albeit without physical form. Christians do the exact same thing with those that still have physical form.
More to the point: in the same way that the difference between the Creator and the created is beyond fundamental, so is worship. In ANE religion-- Christianity included-- you don't "worship" by prayer. You worship by, fundamentally, setting up an altar to a god and offering said god something. For Christians, they offer bread and wine to God on said altar, but unlike other ANE religions, that bread and wine is understood to be transformed into the body and blood of Christ. Additionally, there are specific things that are said of God that inherently cannot be said of a created being, and would constitute idolatry if said of a created being (as well as incorrect).
The Trinity (the father, the son and the holy spirit) makes it also difficult for me to see Christianity as strictly monotheistic.
There's no value in modifying "monotheistic" with "strictly". A religion is either monotheistic or it's not, and Christianity insists that it's monotheistic because the triune God they worship is to be understood as "one God". Examining the struggles that led to the ecumenical councils further demonstrates that this wasn't a matter of trying to cling to the concept of monotheism for no particular reason, either-- Trinitarianism, especially in the Arian controversy, was the
extreme position compared to the various, sometimes deliberate compromises under the semi-Arian umbrella. There was the opportunity to just consider the Son asymptotically close to the Creator-creation line without crossing over into "Creator" territory, and it was rejected in the long term.
There are examples of Pagan gods becoming Christian Saints
There is if you're trying to troll a Christian that believes in the communion of saints, but not in reality. You may as well say that Jesus was a lift from Egyptian myth.
A lot of Pagan rituals were also incorporated into Christianity to make it easier to spread.
Such as?
There may have been customs that were "Christianized" in order to provide better accessibility to alien groups. Especially in Africa, missionaries would often recast existing mythologies in a Christian lens to the extent that they were able without sacrificing the Christianity.
That's still far and away from just "incorporating pagan rituals", which implies that there's still pagan import to those practices.