I think history is important here.
When the .357 Magnum cartridge was developed, S&W chambered their N-frame for it. Originally as the Registered Magnum and later as the Models 27 & 28. That was wise in terms of strength but less than ideal is terms of a carry gun. If you ever open a model 27 and look at those relatively small chambers (S&W calls them “charge holes”) you will see there is a LOT of extra metal in that huge cylinder. And that huge cylinder required an equally huge frame to contain it.
When the magnum K-frames were introduced and the conventional wisdom was the gun would be predominantly shot with 38 Special rounds but could occasionally fire .357 magnum rounds, the concept worked.
The Ruger DA Six’s (Security-Six, Service-Six and Speed-Six) were designed from the ground up as .357 magnum revolvers. The DA Six series guns were the size of a S&W K-frame with the strength of a magnum revolver; and the Rugers cost less. That was 1972 and S&W dominated the market, but they now had a real competitor. Ruger never knocked S&W off the top of that hill, but S&W now had to fight to hold onto market share.
Around the same time, training was evolving to require the use of duty ammunition which meant more magnum rounds were being fired AND there was a trend towards lighter (meaning shorter) bullets. Those factors combined to accelerate wear on magnum K-frames. Cracked forcing cones, stretched frames, battered rachets and excessive end-shake are real things and I’ve seen them. The magnum K-frames are excellent guns, they just were never intended to be used that way.
S&W knew they had to address the issues of the magnum K-frames and in 1980, they introduced the L-frames.
Colt was alive in the 1970’s and 1980’s but I wouldn’t say they were a real threat to S&W in the LE market. Yes, there were some die-hard Colt agencies out there, but S&W consistently won more LE contracts. Individual officers and deputies purchased Colts, but agencies tended to purchase S&W or Rugers. It was mostly an issue of cost.