Not all tentpoles have the budget of Avatar. It is still in the top four P&A spends if all time, the other three all go to Chris Nolan pictures.
The opening weekend thing is a bit misleading. What happens is that the studios/mini-majors push for a strong opening weekend to create the biggest momentum and hype. If there are bad reviews ( and New York and LA reviews can sink a film) they can counter them with a media onslaught of hype and free coverage, based on the opening weekend and usually up to the next 3 weekends. If the movie looks as if it can sustain itself at the box office the studios will continue to spend and sometimes increase the P&A. If the movie cannot cover the 'house nut' the cinema will pull it. This never happened with Avatar and it generally doesn't happen with the tentpoles until month three. Of course, there are exceptions. The point I'm making is that the studios try to predict what will happen with the movie over the coming weeks where as the cinema makes a decision on sales. Therefore, there is a lot error in the decisions of distributors at this stage and someone who wants to be safe and does not fully understand audiences, can end up making a decision which is harmful to the film. This is especially true when higher level bosses, who are not directly running distribution, rely on revenue patterns to try and predict the optimum rate at the P&A balances with the revenue. You would think that they would keep going as long as theatrical revenue is coming in but they do not make their biggest profit in theatrical, it always makes the lion's share of profits from the ancillary markets. Currently, Blu-Ray and VOD deliver the best margins. Most distributors see the theatrical as the best advertising campaign to sell VID/Blu-Ray, etc.. So, it is not that the movie has its best chance at the opening weekend, it is that the distributor bases his decision on that weekend the next three, skewing the importance of that weekend.
What Avatar had, and First Knight and Inception had too, was a head of distribution who understood that there is life after the opening weekend. They learnt from films with (manufactured) word of mouth and created teams of people to do this. They also had a huge P&A which they put in on the basis of Cameron's success with Titanic. Cameron wisely involved 3D, VFX and other film skill bases from numerous countries ensuring some level of free and committed support from within foreign film industries. They also planned ahead for the digital and 3D drive. Most European and Far East nations received government support for converting to digital and for 3D screens. Cameron, and it was his idea, arranged for half hour teasers for the owners, funders of 3D screens and the manufacturers of the technology. They were persuaded to hold the film over for a much longer period than the usual run and between them they devised a campaign whereby Avatar became the ad campaign for 3D. Most of this was funded by government grants combined with money from the other 3D participants. So, that money should really be added to the cost of marketing the film as an indirect cost.
There was always a plan to keep Avatar on screen for a long time. There was new marketing material available for each month of the campaign: there were pre-planned, pre-scripted interviews, tours, behind the scenes footage; there were tie-ins (the Coca-Cola one was huge), competitions, all planned at least a year ahead. Nothing they did was an accident. It was a mastercass of marketing. The great thing about Cameron is that he can think. Quite often producers look at numbers and patterns and don't understand behaviour. Cameron does.
You can do this for indie films too, see Slumdog Millionaire, but because the P&A is lower you won't reach the same heights as Avatar but you can make that money work smarter. Slumdog sustained a year long top ten position in the international top ten, despite having no names, because they used some similar tactics (minus 3D of course).