First let's recall. We have at times had some significant changes. Some new powerhouses emerging and staying there.
1998 saw McLaren emerge at the front with only Ferrari keeping up. McLaren stayed in title contention for three years before dropping off, leaving only Ferrari as the top team till 2004, only occasionally challenged by others (2003).
Then 2005 saw the rise of another top team, the Renault, who stayed there 2 years before the pullout of Michelin tyres.
Then for two years (2007-2008) the old lead-combo of McLaren and Ferrari stayed at the front.
2009 saw the emergence of new top teams. Firstly, Brawn, whose case was very unique and they didn't stay there long. But more importantly Red Bull, who stayed there all the way till the end of 2013.
2014 has seen the emergence of Mercedes. Now - unless the Stuttgart HQ decide that they have had enough of F1, there is no reason to think Mercedes is likely to drop off anywhere. They have got everything - advanced (hybrid) technology, relatively newly hired very talented engineering team, infrastructure, budget. They are a proper top team.
It doesn't mean Mercedes is going to have all smooth throughout the years. They can have some difficult periods, like Ferrari in 2003 or Red Bull in early 2012. On occasion they may be beaten others. They may not always win championships. But they are there to be at the front and at the very least fight for the title if not win it. For many years to come. Until when? Until the next big regulations change? When could that be?
Mercedes is dominating now. But let's take a long-term view. Is Mercedes the new F1 big powerhouse, which will go into the history like the Williams of the 90s, Ferrari of 00s, Red Bull of recent years? The important thing is - They have got the ingredients.