Some older weapons are not replaced by new designs because the older stuff still gets the job done and the users (the troops) prefer it. Case in point is the BGM-71 TOW (Tube-launched, Optically-tracked, Wire-guided) ATGM (anti-tank guided missile). In early 2018 Saudi Arabia ordered another 6,696 TOW 2B missiles at a cost of about $100,000 each. TOW has been in service since 1970, and over 650,000 missiles have been manufactured. All versions are shipped and fired from a sealed launch tube. The 1970 version weighed 19 kg (42 pounds) and had a 3.9 kg (8.6 pounds) warhead. The latest version (TOW 2B or BGM-71F) weighs 22.7 kg (50 pounds) and has a 6.2 kg (13.5 pounds) warhead that can defeat ERA (Explosive Reactive Armor).
Raytheon (the current manufacturer of TOW) and Thales are investing over $30 million to develop new components for the TOW missile that will keep the system current, more reliable and cheaper to build and maintain. This sort of work has been going on for over three decades and is one reason why TOW is still widely used after all that time.
Despite having been in service since the early 1970s, the TOW missile has turned into another one of those perennials. In other words, a design that is so good it is difficult to replace and the original continues to be useful and in demand. Sort of the like the Sidewinder air-to-air missile and the M-16 or AK-47 rifles. There have been many new and improved competitors developed, but the originals (somewhat upgraded) continue in service, production and demand. There are so many TOW launchers and missiles out there that it has become big business to refurbish and upgrade both launcher and missiles. That is a lot cheaper than buying new missiles or missile designs and with TOW you know what you got and are comfortable with it.
The last time TOW destroyed tanks in large numbers was in 2003, during the Iraq invasion, but it was since been used frequently against enemy strongholds in Iraq and Afghanistan. There may have been some recent tank kills in Syria, where the rebels have received some TOW systems from the United States. TOW has gotten high praise from operators throughout its four decades of use and appears to have a decade or more of life left in it, at least on the ground. In the air, TOW has largely been replaced by Hellfire, which came into use in the 1980s and has undergone several improvements. There are also several more recent and smaller missiles that are displacing Hellfire. TOW was innovative for the 1970s but has not been able to evolve fast enough to eliminate the market for new designs.
One thing that distinguishes TOW from later designs is that more recent missiles are wireless. This has not proved to be as critical an innovation as many thought. There have been several wireless versions of TOW. Raytheon's radio controlled TOW was developed for use on AH-1 helicopter gunships, and the Saudis bought over a thousand of these wireless (RF) TOWs for ground use by their National Guard (a tribal militia formed to protect the royal family). There were other wireless TOWs. Work on such missiles dates back to the 1980s. But the U.S. Army never adopted any of them. Israel developed its own wireless version (MAPATS or "Laser TOW") in the 1980s. The Israeli TOW uses a laser designator and still has a range of 4,000 meters. MAPATS weighs 29.6 kg (65 pounds) and evolved into a different missile in the 1990s. The Raytheon wireless TOW was lighter than MAPATS but still had a range of only 4,000 meters.
The thing TOW has going for it is reliability. The wire guidance never became a problem and with the advent of electronic countermeasures against ATGMs, TOW finds itself immune to those countermeasures. TOW gets the job done, with either the wire guidance or later wireless models. It is a simple, precise and relatively cheap weapon that has constantly proved useful in combat.