Israel and the United States are two of the main users of the Vietnam era M-4 carbine (the smaller sibling of the M-16). The United States has dwaddled in the procurement process as Baghdad has burned, but is looking for a replacement. Israel, while not exactly expeditious itself, has decided on a replacement, the Tavor-built TAR-21.
I can't tell you in any intelligent way why one might prefer a TAR-21 over an M-4, nor can I tell you why Israel made its choice, although I am comfortable in believing that for all the corruption found in parts of Israel's government these days that small arms procurement was not deeply affected by this problem.
I likewise have no expertise to the judge merits of the ongoing debate over whether the U.S. needs to replace its small arms, and if so, with what, although soldiers and gun afficianados never tire of the exercise. But, I am aware, from reading many of those debates, including the one in the comments to the linked post, that there is a great deal of dissatisfaction with the status quo in the U.S. among many parties.
The trouble is, of course, that dissatisfaction comes in multiple varieties.
Some bemoan the NATO standard 5.56 mm caliber of the M-4 and M-16, arguing for a larger bullet (usually 6.8 mm to 7.62mm, and sometimes even 12.7mm) with more stopping power, but generally also with more kickback. Others argue that 5.56 mm has proven more than sufficient to stop and kill opponents.
Some argue that shorter rifle barrels are better for cramped urban combat and vehicle duty, while others argue that a large rifle barrel is easier to aim.
Nobody is arguing for a less reliable weapon, but some believe that increasing reliability is an important priority, while others feel that this isn't a major concern, or that an array of small arms that reflect a diversity of combat environments is necessary to achieve desirable reliability.
My contribution to the debate is to argue that to the extent that better options than the status quo are available (even a mere upgrade, and I don't think that anyone seriously doubts that proposition, that we do our soldiers a great disservice by not refusing to pick one of them, even as an interim measure. Small arms are a drop in the bucket compared to overall procurement costs, yet even a slight beneficial impact at the critical place where soldiers live and die in our existing conflict would be worth it. The fact that we purchase a better option now, which isn't the best, doesn't preclude us from purchasing an even better one in three or four years.