I'm sorry but I genuinly don't see how it made it more of an organized religion.
That religion was institutionalized is out of question, and probably was as far as the IXth century (basically the time for western slavic peoples to stabilize), but the presence of an huge sanctuary doesn't proove the existance of an organized religion, as in a religion with a systematical organisation and codification, critically over not only one tribe but above all western slavic ones.
Have we, by exemple, a priesthood hierarchy distinct (physically or not, roles possibly being merged on one individual) of tribal hierarchy? Basically a religion independent of tribal features.
I tried to found some in Saxo Grammaticus, but I don't read Dane (and translation of the books that interest me aren't avaible). That's, again, a genuine question.
(For example, Delphi was a pan-hellenic sanctuary, but it didn't made Hellenic religion more organized in the strictest sense, as in an unified theological and institutional body)
You have a point, maybe it isn't a very good indicator, but it seems to me that there would need to be some organization for the Rani's temple in Arkona to become the major center of worship. And not just for the Rani, if I recall correctly, but for all the remaining pagan Slavic tribes in the area.
I just think that if all the Western Slavs (Poles, Bohemians, Wends, Sorbs) had a couple more centuries or so for their pagan religion to develop, it's possible they could have avoided converting to christianity.
Although the biggest thing that would have helped them avoid that was if more German tribes had remained pagan as well.
As with many wars back then, one key battle could have changed the outcome entirely. It may have convinced Charlemagne that it wasn't worth expending anymore resources on Saxony. Or perhaps if Widukind had convinced the Danes to join him in fighting against the Franks. Any number of things could have changed the outcome of those wars.I'm not sure it would have been possible. Saxon Wars themselves were easily won, the difficulty being to hold the country. With the full strength and ressources from Gaul and Italy, and the Saxon division, it would have been hard.
If you allow me to twist a bit the situation, a sucsessul conversion of Frisians and Saxons by Anglo-Saxon missionaries under Frankish watch would be interesting. No doubt that at the first sight of rebellion, they would know the same fate than Alamans, but it could make Frankish expansion in Germany less brutal and less deep.
At best, the Wendish tribes could avoid being in the Carolingian sphere of influence, and maybe prevent being crushed by Danes in the process.
I don't think it would have prevented christianization of Europe as a whole, but if it could have delayed it enough, maybe the smoother and slower Christianisation of Germany would have provided both a reaction among Scandinavian and Wendish peoples to organize their rites along a more unified body, imitating Christian features.
And of course, develloping their own practices along : you could end with 4 or 5 allohistorical equivalents to Lithuania up to the XVth century.
But you have an interesting idea there too. A slower, more peaceful conversion of the Saxons could have changed things. It seems to me that the Saxons were quite stubborn, so it might've taken a very, very long time, or they may not have ever accepted conversion through peaceful means at all.