Laos is a landlocked country which need connections. By connecting to China, Laos get a 14 billion market.Laos is only a 7 million country. China can offlload some labour intensive , low-value industry to Laos to give it kickstart.
This industry would then promote internal circulation and eventually Laos would have accumulated enough capital to start working on its own
I don't think Laos is in such a big trouble. Obviously the biggest obstacle to China helping it develop was that it lacked the railway connection. Now with that done, expect more businesses to be expanded and created in Laos from 2022 and onwards
The real question is why Thailand is dragging its feet on the railway to China... Their neighbour has just completed its railway link to China, while Thailand is now ridiculed for their tardiness. Laos may become a transportation hub if Thailand keeps stalling
Thailand has lots of sea rout, it doesn't have that much desire compared to Laos. The price tag for HSR is high. Although it is also high for Laos, but for landlocked country that connection is more of a must have than good to have (for Thailand).