When top tier ISPs are still limiting speeds to 100Mbps, what exactly is the need for modems that transfer 4000 Mbps?
In most places I've been over the last few years I deal with 60-70 Mbps bandwidth, which I can almost hit on an antique 802.11g card. The only change I notice between g/n/ac and wired is ping, and between the wireless cards it's a negligible amount.
I've been thinking of upgrading my old Lynksys n750 but I don't see a point. I have multiple phones, a laptop, game consoles, printer, and a Blu-ray player going all day and it doesn't hiccup. The only reason I've considered something with more bandwidth was to setup storage on the network. So unless you're moving data within the house on a wireless network I don't see the point, but I would more than happy to hear reason.
For a long time I thought the same way you did. If all someone had was 25Mbps service, all they needed was an 802.11n router. But there are a few reasons you might want a faster router even with mid-grade internet service:
(1) Range: it's not just about whether 802.11g or 802.11n or 802.11ac has the longest range based on the laws of physics. It's about the power of the router. My Linksys EA9500
has substantially greater range on the 802.11ac 5GHz band than lower-end routers have on the 802.11n 2.4GHz band. Basically, new high-end routers have grown in size and processing power to overcome the inherent range disadvantage of 802.11ac networking, which has provided the winning combination of 802.11ac's huge speed advantage along with an increase in range.
(2) Multiple radios: while you may have had luck so far with your Linksys N750 rotuer, newer routers with multiple radios are much more adept at handling modern usage patterns. You can dedicate the 802.11n radio to lower-bandwidth devices like phones, printers, and smart home devices, while having an 802.11ac radio (or two) used exclusively by high-demand devices like TVs, HTPCs, tablets, and game consoles. That allows more of them to be connected simultaneously.
And as an aside, while it's true that most DSL companies have capped their service at 12Mbps and most cable companies have capped their service at around 50-125Mbps, that isn't the end of the road. I'm on Gigabit fiber, and more and more communities are seeing fiber arrive to compete with the entrenched DSL/Cable companies that have had little incentive to increase bandwidth.