Roshi has been training in secret the entire time

[Yasai9001]

It dawned on me that after the 21st Tenkaichi Budokai, Roshi admits during the 22nd one that he had been doing secret training in preparation; later on we see his power-level is at 139 and people have believed that to be his power the entire time. I heavily disbelieve that to be the case. Roshi has likely been training still, but just remains weaker compared to every other fighter. His 'skill' and 'expertise' is what helped him in the Tournament of Power. Being a Master Martial Artist and being the one who practices what he preaches, why wouldn't he keep training? Roshi firmly believes that there is always someone stronger - he has admitted that the next generation is leagues ahead, but why would he stop continuing his training?

On top of the fact that he was able to take on Freeza's goons later on, MOST whom have power-levels ranging from the low hundreds to the very low thousands. He kept up and defeated a good bit of them.

What do you think

[Benjamin-Simons-91]

If you ask me, I have Roshi in his debut at about 70-75 when he went full power.
Then by the 21st Tenkaichi Budokai, he's at 100. Next at the 22nd TB he's at 139 when he's at full power and stays in this plateau until Raditz shows up. That's why Kami didn't invite him to train under him and Mr. Popo. He seemed done.

Now, following his "renaissance" in Dragon Ball Super, I believe that his power level almost always doubles from saga to saga to keep him relevant:
* During the battle with Vegeta and Nappa his power level at base must be at the 300s.
* After the Frieza Saga his base form power level must have rose to the 400s.
* When Cell attacked his base form power level must have doubled to the 800s.
* When the Majin Buu Saga happened his power level must have doubled again to the 1,600s in base form.
* Then, when Resurrection 'F' happens, his power level is at the 3,200s when he's at base form and at the 6,400s when he's at full power - as he can take on 177 of Frieza's second rate soldiers.
* In the Tournament of Power his power level should be at least 12,000 in base and at least at the 24,000s at his buffed form. I omit the Yurin episode as it makes zero sense that he'd be above Tien nor excite Goku who is in the billions for sure at base.
* By the Galactic Patrol Prisoner Saga his base form power level must be at the 48,000s.

[Yasai9001]

If you ask me, I have Roshi in his debut at about 70-75 when he went full power.
Then by the 21st Tenkaichi Budokai, he's at 100. Next at the 22nd TB he's at 139 when he's at full power and stays in this plateau until Raditz shows up. That's why Kami didn't invite him to train under him and Mr. Popo. He seemed done.

Now, following his "renaissance" in Dragon Ball Super, I believe that his power level almost always doubles from saga to saga to keep him relevant:
* During the battle with Vegeta and Nappa his power level at base must be at the 300s.
* After the Frieza Saga his base form power level must have rose to the 400s.
* When Cell attacked his base form power level must have doubled to the 800s.
* When the Majin Buu Saga happened his power level must have doubled again to the 1,600s in base form.
* Then, when Resurrection 'F' happens, his power level is at the 3,200s when he's at base form and at the 6,400s when he's at full power - as he can take on 177 of Frieza's second rate soldiers.
* In the Tournament of Power his power level should be at least 12,000 in base and at least at the 24,000s at his buffed form. I omit the Yurin episode as it makes zero sense that he'd be above Tien nor excite Goku who is in the billions for sure at base.
* By the Galactic Patrol Prisoner Saga his base form power level must be at the 48,000s.

And you know something? None of this seems bad at all - let me explain why:

Time and time again, throughout Dragon Ball, we see there are moments where people feel as if they've reached the limit of their power/potential, but once they surpass it, others can, too. Key examples would include Goku after beating Piccolo Jr, Roshi realizing Tien and the others are far ahead of him, Vegeta realizing that there is a layer behind the Super Saiyan form, etc.

I want to highlight everyone reacting to Raditz's insane power; he was effectively five times stronger than every other person in Dragon Ball excluding Piccolo and Son Goku and maybe Kami at bare minimum. After seeing such a power, the Earthlings find their benchmark and surpass him. Because of the fact that Son Gohan from Trunks' future timeline never got to see how to master the Super Saiyan form, he kept losing and ultimately died. It takes for characters in Dragon Ball to see others pass their own limitations to soon realize that there are so many different walls of limits that they can break themselves. Roshi has been there from the absolute beginning and has seen time and time again how so much stronger everyone else can get (Saiyans, Namekians, Earthlings), and likely realize he had many more walls he could push through, too.

Roshi has always had less raw talent than practically everyone else in the story shown, but he's still a Martial Arts master at heart. He is old and more intune with his spiritual and mental strength than just raw power, much like how Piccolo is. I don't see why anyone would gripe over Muten Roshi becoming so much stronger; he's an Earthling, too for one, and for two, he never mentioned how he stopped training in private. Seeing anyone like Goku, Krillin, Piccolo, and Tien reach new heights time and time again probably motivated the old man more than anything.