My guy, this is a fictional story. Oda can easily write them being able to find Dragon location just like how BB just conveniently was able to find Law. Also, who gives a flying fuck if Dragon is powerful? World government is powerful too, who cares how powerful he is. How weak is the WG if they aren't even willing to go after their top enemies. That is terrible writing to make the WG that weak and pathetic.
And you keep showing that you have no clue of how reality works. We have lots of examples of weaker groups of revolution that powerful governments couldn't find, catch and kill just because. Oda would have it easy to write Im having a heart attack and dying for the Government to collapse; but he isn't doing so no matter how fictional his story is because that's bullshit, just like cornering and defeating the whole Revolutionary Army just because they feel like it.
At this point they know they can't just go after Dragon and his people and obliterate them, and this isn't (just) a matter of strength.