That may be so, but I don’t accept the premise that flat, minimalist design is a current standard, or at least not a good one.

The current trend for flat design is a triumph of mediocrity. It is what happens when using more varied styles and original graphics and designing to match the content becomes too difficult, time-consuming or expensive. This could be because of having less than ideal graphics software or formats available. It could be because modern sites and apps often need to support a wide range of output displays. Frankly, I think a lot of it is simply laziness or incompetence.

Whatever the motivation, designs started getting dumbed down to what you could do trivially with CSS, and often even further to what you could do trivially with someone else’s CSS framework. It’s all rectangular blocks, and straight lines and circles done with borders and transforms, and web fonts, and monochrome icons implemented using web fonts, and maybe a bit of animation. But that means everything flat is the same: bland, no brand distinctiveness, and often with glaring usability problems. After a while mobile apps and mobile operating systems jumped on the bandwagon too.

The absurdity may have finally reached its logical conclusion, though. I actually read an article the other day — a serious one, though you’d be forgiven for thinking it was published by The Onion — where someone from a big name firm made the insightful observation that having a drop shadow on a button could make it easier for users to identify it than... just using completely plain text. Truly a remarkable discovery, and a radical step forward in UI design that brings us right up to... the 1980s. ;-)

As you’ve probably figured out by now, I’m not a fan. IMNSHO, flat design is the worst trend we’ve had for as long as I can remember, and not just because I happen not to like it as a trend, but for objective reasons like the difficulty in being distinctive and the poor affordance of interactive elements. You mention dated standards that have proven ineffective, but take a look at the comments on iOS 7/8 and Windows 8 and then tell me with a straight face that Flat Design is effective.

So if your client wants to do something in a style that is dated and seems unlikely to get good results, as a professional I think it’s fine for you to diplomatically point that out and suggest alternatives. But please don’t just suggest flat design instead because it’s trendy, particularly if your client has made it clear that they don’t like it. Trendy and good are two very different things, and as professionals we should do better for our clients than what their thirteen-year-old kids can whip up in ten minutes with Bootstrap.