He's had an injury disrupted season this year but he's one of our most productive players.
As always, completely disproportionate comparison, Nani didn't arrive at Utd as young as Ronaldo or get played in the same way, would you expect Ronaldo's first 100 appearances from 18-21, to produce the same as the second 100 from 21-24? Because that is basically the comparison being done.
Nani is about 1.5 years younger than Ronaldo, yet joined 4 years later, he had 76 appearances and 11 goals to his name at Sporting, Ronaldo had 31 appearances and 5 goals, his first couple years weren't particularly good, he was adjusting to the league, to a new country and doing it at a younger age when you don't really expect a huge amount from a player.
Nani's best season in the league to date is 9 goals 14 assists, Ronaldo's best at Utd alone was 31 goals, 7 assists.
Now factor this in, Ronaldo had that best season 5 years ago, and has put in 4 brilliant seasons since, he's only 18 months older than Nani, his best season was 2 seasons ago(in terms of stage of career and age, WELL after Ronaldo) and he couldn't match that this season.
Compare that first 100 appearances of Nani's to Ronaldo's first 100 at Real, it still fairly pointless, but a lot more relevant than comparing what Ronaldo did from 18-21, to Nani from 20-23(ish).
This also ignores that fact that from 2003-2006 Utd had Arsenal and Chelsea winning the title, and Utd were simply a weaker team when Ronaldo arrived, and in no small part due to Ronaldo coming of age after his first couple seasons that Utd got so much stronger and were in a much MUCH stronger period when Nani arrived.
Is he worth keeping, yes, is he one of your best players, yes. Neither of those things adds up to automatically offering him any money to keep him. LIkewise almost everyone has agreed Hazard isn't worth the fee nor ridiculous wages, he's unproven in a top league though looks very promising.
For me, I think Nani will want out of Utd at some stage anyway, so you can throw very high wages at him(when the club realistically should be looking to get rid of debt as quickly as possible for long term complete security), then see him leave anyway. The other option is let him go now and look for someone as good or better. He's rarely put in "long" seasons so far, he's only started 25+ league games twice in his career, he's only played in over 30 games in the league for Utd once.
Just because City and Chelsea can throw completely stupid money at wages and take 200mil losses a year doesn't mean anyone else should. 130k at a non billionaire backed club is for simply incredible players, not just pretty good. For a player who is frequently annonymous, is only sparingly brilliant, does very little work defensively, and isn't the fittest player around, and at his best is some way short of being the best winger around. None of that add's up to throwing money at him, if he'll stay on a reasonable contract, sure, if you can sell him and bring in a fantastic promising younger/cheaper player, that's just good business.
If he was going to turn into a Ronaldo, 20+ assists every year or 20+ goals, he'd have done so years ago.