The first bit's not always true. It works provided that one doesn't hinder the other - the classic example being infantry divisions tied down when their armoured comrades ran out of fuel in the days before mechanised infantry.
True, but we buy tanks to reconnaitre (which Apaches do better), to move quickly (which Apaches do better, to provide us with firepower specifically against other tanks (which Apaches do terrifyingly well) and just for their psychological effect, and there are reports of enemy positions in Iraq surrendering at the sight of an Apache. They also completely ignore most terrain, lend themselves naturally to ambushing, and can be flown from a helicopter carrier - as well as being notably more difficult to kill. Obviously it would be very difficult to replace the Royal Armoured Corps with them overnight, but it's increasingly difficult to see a compelling reason not to slowly replace conventional armour with helicopters.
I challenge this view that because two weapon systems have interchangeability of some roles, they are therefore completely interchangeable as weapons. The Apache is not a replacement for the tank and no army in the World is planning to replace its tanks with helicopters.
There is a massive price tag difference between Apaches and tanks, and different crew requirements [note that an Apache needs a ground crew for refuelling and maintenance, as well as some form of elementary air base]. With Apaches retailing at anything up to $100 million a piece, there are severe restrictions on the number of these weapon systems that can be employed by any army.
The Apache needs air protection when travelling to and from its mission area, and if air superiority is lost locally or strategically, the Apache is vulnerable to air strikes on its fields or to intercept from air superiority fighters. Tanks can disperse into light forests or urban areas and have other means of operating when air supriority is lost, whereas helicopters will go down when their air superiority umbrella does.
Also, there has been mechanisation of ground artillery, air defence, infantry, engineering, logistics and C3 in all modern armies. These mechanised ground forces manoeuvre best with supporting armoured fighting vehicles that can remain close to them and form an integrated combined-arms team. Helicopters are not able to remain on-station and dig-in or manoeuvre with these formations the way that tanks can. Especially as helicopters are often held at the corps level and assigned to different missions that may take them away from the division in combat.
Thus, replacing the tank with the Apache would weaken a force considerably. The Apache adds a new dimension to the combined-arms team but it definitely is not designed to replace the tank. Perhaps in an expeditionary force that is acting as an imperial police force, but not in any serious army built for high-intensity modern warfighting.