lol that is exactly what I thought. I was eye rolling when Lah got the money.
So I tried to look at the story differently. I rationalized with myself that Tayyang represents "opportunity" He says it often that if the right person gets an opportunity, they'll be able to make something of it like Lah.
Tayyang gives Lah just enough money to start a business. He tests him to see if he has what it takes to become a partner of his. For Lah, it was never guaranteed. But as viewers, we never really saw a threat to him not getting the spot. So I know what you mean.
Like if you were Hiran with basically no skills, he wouldn't be able to make anything of any opportunity he gets. He grew up with a silver spoon and has every opportunity in the world but isn't able to make anything of himself like Lah with less. Or someone who is greedy but isn't willing to work, they'd just squander the money. For Lah, he already has that work ethic and being able to get along with people and motivate them to work towards a common goal. With Fah, same and she's smart.
That opportunity could be anything. Getting a better job, getting the right connections (i.e. Tayyang), entering a contest (that's prevalent in Thailand), etc. Not everyone is going to get life-changing opportunities of course, but you do need to have that foundation to make something of it if it does come knocking. I think that is what the story is aiming at or at least, I hope so.
Overall, I agree though. The writers could have gone at it differently with something more realistic like Lah working for the money to open the business himself and Tayyang appearing later on maybe.