Well, here we go.
Episode 50: For the most part, I liked it. Zamigo's finale battle was well savored, though as Dr Kain pointed out, it would have been nice to have some more battles like this during the earlier episodes. Still, you gotta admit, with a visible hole in his chest, Zamigo's defeat was pretty brutal... Perhaps the most brutal thing in the whole series!
And wait, since when can Trigger Machines change the combinations of the safes?! Yeah, I understand why they didn't save that info until later. Dramatic reveal, and that ability is probably useless for the most part. But still, it would have been nice to see that ability
JUST ONCE midway through the series. Maybe put in a filler episode so that people easily forget about it.
Episode 51: The finale was pretty awesome, honestly. We got to see the police changing things up by using the Dial Fighters. We got to see the Phantom Thieves making the ultimate gamble that put their lives on the line. And the most awesome thing of all... THEY DIDN'T DOWNGRADE THE FINAL BOSS! Super Sentai has a VERY bad habbit of saving the final enemy for last, making them look tough at the beginning and midway through the series, but then making them weaker because the heroes have come a long way or something. Here? Not the case! They had to teleport the Collection Pieces out of his chest to make that happen. Nice! I can even forgive things like Kairi/Lupin Red suddenly having his own book and Jackpot Striker suddenly having mind-control powers. We're wrapping things up. I actually cried a little bit during the sentimental moments, cheered during the happy moments, and overall enjoyed it. Does it save the series as a whole? No. But there are other things to be happy about.
...We now have Jackpot Striker's magical powers that can also bring the Thieves back too?!!?
[Snip!]
Wasn't their mission to save their loved ones, mission complete, go home and enjoy life. Why continue with the collection? It doesn't make sense.
As for Jackpot Striker, I don't know what happens in the movies, but read above. According to Over-Time's translation, Jackpot Striker can control a Gangler's mind as long as he is in their safe. So they transported Jackpot Striker into Dogranio via Kairi's book, and forced Dogranio to release the Lupin Rangers and Jackpot himself.
And the show answered your other question. By collecting all the pieces of the Lupin Collection, you can have a wish magically granted, maybe. So while the thieves really don't need to be rivals with the police anymore, it's important that they collect all the pieces, in case a remnant of the Ganglers (or someone just as dangerous) tries to make a wish instead. The ending makes... some sort of sense. It's flimsy logic, but logic none the less. I think the ending was mostly there to get one more group shot of the PatRangers and the LupinRangers together. (Personally, I think they should have tried to recreate the fight/robbery from the opening sequence.)
My thoughts on the series as a whole: At the beginning, I made some predictions about this show. As the series went on, more predictions were made. Many of these predictions were proven false, but some were proven true. Some people expected Kogure to be Dogranio in disguise, but that didn't happen. My biggest prediction was that the LupinRangers and PatRangers were going to discover they were always suppose to be part of 1 team instead of 2 divided teams; but I'm actually kind of glad that didn't happen. Props to Toei for sticking with their guns (no pun intended) and keeping them two different teams. It makes the ending completely fair to the fans that did pick a side (including the kids). Combining them both into one team would have made their "Whose side are you on?!" promotion of the series completely pointless. This way, both sides come out on top, because now it's a friendly rivalry instead of bitter enemies.
That being said, the series is still full of problems. A show centered around two Sentai teams facing off against each other? This should have been the series where Toei was willing to experiment, take chances, and do things that keep our attention every episode. Instead, for the vast majority, they just play it safe/repetitive. Many of the big villain characters left little impact on the show in the long run. Epic moments that the fans had been waiting for were saved for the last possible moment instead of happening sooner. And a lot of potential was completely wasted. I won't say it's the worst show; I've seen fans who enjoy it regardless. I just hope Toei really takes us by surprise with the next series.
Now, if you'll excuse me, I need to see a historian about some dinosaurs...