No. Just one big no. I am not going to be grateful. Nothing was given to me. They weren't like, hey its that dude, lets give him a win because he is awesome. No way. I have been on the side with leavers. It is usually because everyone is tilted on that team. It could even by the last person who is left that caused their team to leave. I hate leavers and people who throw fits and cause people to leave. It 100% should not be rewarded with kindness of any sort.
The goal is to have as many quality 6v6 matches as possible. Anything that gets in the way of that is just nonsense and absolutely does not belong in competitive. Why do you want to be counterproductive to that? Why do you want to make the Competitive experience worse? I just don't understand this. Yes the game is supposed to be fun. But honestly it is way more fun when an actual match is happening. I don't give a fuck about the one person preventing 6 people from playing another match. I don't care if they think I should be grateful. I don't care that they want this or that achievement. I just want to keep playing quality matches. Once more than one person leaves, it is obviously not a quality match anymore in almost every instance.
I would rather have a match where I get steamrolled and I tried vs waiting for a match to end because some dude thinks he is going to save him/herself 3 or 4 SR. That simply isn't true, you are making it up in your head. If you think it is true. Track it. Oh wait, there are less leavers than people think there are, there is no reliable evidence to suggest that you lose more SR if you leave earlier. Absolute nonsense.
You know what you SHOULD be grateful are? Flex/fill players that let you play the hero that you think you are good at. You should be grateful for Lucio mains that don't think they are DSPstanky. You should be grateful for people that understand what a tank is supposed to do. You should be grateful for people that are coming into a match ready to win by playing as best as they can. I will never be grateful for winning by default.
You are what is wrong with Overwatch if you think Competitive is a place to be grateful for an easy win. The developers try their best for fair match ups that create a competitive environment. To deprive people of starting another match in hopes to have a quality match, is absolutely rude.
The only reason people will make friends with the lone person is because it passes the time. You want an achievement, sure here you go, because I AM BORED. Nearly everyone who is waiting for the match to end would rather you just leave.