The answer to this won't be what you're expecting. Complaining is relative and there was no less of it then than there is now, but what has happened is that what the fanbase complains about, and how certain it is to make up its mind and be adamant it is an issue, carries a heavier tone as the dissenting voices are vociferous, with good reason.
Ten years ago takes us back to 2013. As a fanbase, we'd have been absolutely used to - and expectant of - domestic success and our aspirations would most certainly have been to lock horns with the best in the world in pursuit of more Champions League trophies. Dissenting voices against the manager himself would have been drowned out, but gripes about playing style or team selection would have been aired, not en masse, but noticeably enough for talking points to be discussed. Issues with the Glazers abound, but Ferguson still winning despite his hands being tied, and quelling upheaval would again keep the lid on anger and annoyance. With Ferguson at the helm, gripes could only be small and there was merit to him figuring things out, ergo, doubt could never be absolute.
Now? We've seen everything enough times over to be stuck in a Groundhog's Day loop of misery and despair. Relative to the glory days of the aforementioned Ferguson era, these are the darkest times some of the fanbase have ever experienced, and even for those who grew up with or before Fergie's era, this is a different kind of uncertainty because it's a team and collection of individuals that are hard to identify with and a club that is harder to connect with and back than it ever has been. Not only has the football been bad, what the club stand for and the values most were raised on are being eroded.
The Post-Ferguson era has also ushered in civil war(s) amongst the fanbase; people selectively picking managers - and players - to back to the very end and defending them to the hilt, whilst giving others extremely short shrift. Such things didn't exist ten years ago; the fanbase was nowhere near as fractured or entrenched in opposing camps - we're the most disenfranchised fanbase in the country; no other club has infighting to this degree. It's been a huge turnaround from the United against the world days that had us despised for our unity and collective certainty of purpose.
The: 'It's been 1, 2, 5 10 games' thing is also wearing thin and so people who once held their tongue and let things run their [obvious] course are more outspoken and certain in their belief that things are wrong because they've seen it all before. Lest we forget it took only two games for LVG to do a complete about turn in how he approached an entire season previously, and just last season, ETH himself succinctly arrested a perceived slide by entering the transfer market after a hammering by Brentford in the second game of the season. If managers can change tact in such short order because they see things are in need of immediate address, it shouldn't be a surprise when a weary, and by now, rather conditioned fanbase does the same. We know more than a thing or two about failure, feck ups and calamitous goings on at varying points in a season. Same goes for players; the benefit of the doubt is not there and there is less credit in the bank or tolerance for failing, but not for failing's sake, rather, in the hope issues get fixed and addressed to avoid the whole script being force-fed over a whole campaign, yet again.
Ten years is a really long time for a giant to be in the doldrums. What is a concern is we rarely look like we're moving forward - if it's not one thing going wrong, it's another, and the net result is we mostly appear inert, certainly when a wider net is cast and analysed. There isn't more or faster complaining than 10 years ago as a whole, but the topics for complaint are rawer, more serious, more certain and more egregious - we've gone from relative 1st world problems of a giant club not doing as well as it should in the Champions League, peered to other giants, to a club with so many issues that you can take your pick of what you wish to be up in arms about at any given time. You're not going to pay much attention to a complaint about 'zombie football' whilst your team is winning the league, compared to the knives being out for, say, Mason Mount before his United career has really gotten underway. What we have now is far more volatility, anger, intensity and frustration; you can say these things are quicker to rear their head and that what was a slowburn in the past in that sphere has gone, but the speed at which complaints are made? No, that hasn't changed, but the topics of conversation have.
Only just finishing inside the top six clubs in the country, oh the horror !No club has a God given right to win things. All we can do as fans is be patient and enjoy the journey, the bad times will make the good times even better.