There is some miss information in this thread about the P229 and its history and how it differs from the P228. The P228 was a mini chopped down P226. The original prototypes where P226s which were literally cut fitted with shorter recoil springs guide rods & barrels. They were of course tweaked from there. They all had alum frames, carbon steel folded/stamped slide, breach blocks etc... They were and are still considered classic P series Sigs.
The P229 was developed as a 40 S&W and .357 Sig gun. It was not created from the P228. It again like the P228 is a chopped down version of the P226. The p226 is the father of both the P228 and P229. When starting to develop the P229 they need to design a different frame because the size of these two cartridges are larger than the 9mm. They could not fit the desired number of rounds into the shortened P226 or P228 frame. Strength was not the issue when it came to the frame. Size and capacity was.
They also developed the milled stainless steel slide which could be produced here in the US. Strength was a major factor but the carbon steel slide could have worked. You could have altered the recoil system, mainly the spring weight, and it would have held up just fine. One of the main reasons for going with the milled stainless slide was that the tooling and equipment used to make the older folded/stamped stainless steel slides are in German they are old and they are less efficient then a milled slide. They were going to have to be replaced or taken out of service. The amount of serviceable life left in that equipment has always lead to the rumors that the P228 was gone forever. The carbon steel slide of the P228 has died many deaths... LOL
Sig knowing that the cost of replacing the older less efficient equipment was hanging over their head so they choose to go with the more efficient milled stainless steel side with the P229. It worked and with faster more precise CNC machines Sig soon started to use milled stainless steel slides across their entire pistol line. The P229 was the first then came the P239, P226, and P220 etc... Economy of scale and the cost savings from CNC machining was really the main factor for these changes. It also allowed them to introduce the 40 S&W & .357 Sig into the P226 lineup.
So strength and economic factors lead to the birth of the stainless steel slide in the Sig line because they needed to bring a 40 S&W & .357 Sig pistol to market. The difference in the frame was also necessitated by the cartridges it was originally designed for. The P229 in 9mm was introduced later as a way to reduce costs due to economy of scale and to have a US made version of the P228 which was due to be phased out of production which seems to finally have happened since Sig is trying to sell us a P228 with a milled stainless slide....