One reason is as you stated, the chips become too worn for easy handling on the tables. However, that doesn't always happen. There are a number of Las Vegas casinos that have extremely worn chips in play for years and don't replace them. They do sometimes reorder the same design chips and put them in play with the current worn chips (Caesars Palace, for example).

However, I don't believe the MGM Theme Hotel casino chip change was due to wear. The original BudJones chips will last for decades, and there was little wear evidenced when they were replaced.

Other reasons for replacing chips are, as you suggested, theft or counterfeiting of the current chip. Also a change in ownership is sometimes (but not always) a reason to recall the old chips ane replace with new ones redeemable by the new owners. Again, many of the Las Vegas casinos didn't replace their chips when they were purchased by Harrahs recently.

Regarding cost, it's my understand that if you are a good customer of the chip supplier and purchase most of your other casino equipment from him, you often get the chips without charge. Doesn't apply to commemoratives, as I underatand it. In any case, the cost of a new rack of chips would be chump change to a big casino today even if they did pay for it.