Until recently, the Remington 870 had about 95% plus of the law enforcement market.
This wasn't because they were cheaper, they were more expensive, it was because the 870 had a 50 year track record of standing up to the police night watch version of "Hold my beer and watch this".
Years ago I knew a large police department armorer, who joked that his 870 spare parts inventory would fit into a shoe box with room left for a burger and fries.
He said that other then damaged wood or barrels he seldom had to do a major repair.
Ultra large amount shotgun clay shooters say the 870 will begin getting cracks near the ejection port in about 250,000 rounds.
You can expect an 870 to go a long, long time without any repairs and those will be small parts replacement.
If there's a shotgun accessory available, it'll be available for the 870.
There are almost unlimited choices in stocks and forends, magazine extensions, sights, etc.
An 870 will have you about as well armed with a shotgun as it's possible to be. There are other pump guns that are very good, but none that are better.