There were quite a few other systems that launched before System X.
The French have been using Alcatel E10 systems since 1972 in their public network as local and transit switches. Full TDM digital stuff since way back then! They've been in use here in Ireland since about 1979.
Ericsson AXE arrived in the late 1970s too. I think BT adopted it in the mid 80s as a competitor to System X to keep costs down by having two suppliers.
In the states Bell Labs/Western Electric 4ESS dates back to 1976 and 5ESS in the very early 1980s.
All this TDM stuff is based on pretty old 1960s/70s era concepts though.
The biggest problem the telcos face now is that the equipment makers are withdrawing support for a lot of TDM switching systems. In Europe they've definitely already begun rolling out alternative technologies at the central office / exchange level anyway.
Moving away from TDM switching doesn't necessarily mean moving away from POTS though. I know for a fact my own POTS line is actually connected to an analogue port on a system controlled by a VoIP softswitch and has been since about 2005.
Just because the network moves to VoIP doesn't mean that dial-tone POTS services will need to disappear.
What's happening here in Ireland though is that all of the FTTC rollout has included VDSL2 gateway modems that include a 2-port VoIP ATA.
Some providers are using them, others are using the exchange-based POTS service. Depends on which packages you opt for and who your ISP/phone provider is, but it works pretty reliably.
Likewise, I haven't had any major issue with UPC's cable phone service which is similarly provided by an ATA in the set top "Horizon" box which combines the TV service, WiFi hub, multi-screen TV server (to iPad app), ATA and voice phone stuff all in a single unit that sits under your TV.
The analogue POTS interface hasn't changed a lot since the 1920s and it's gone through several generations of equipment behind it from simple electromechanical systems, to crossbar, to computerised crossbars and relay systems to digital TDM to digital VoIP.
What I would like to see is a system that could use the exchange batteries to power the ATA/modem/router much like the way power-over-ethernet works. There's plenty of juice in the exchange and the wiring could handle it easily.