Beware of geeks bearing gifts.
I always get dubious anytime a marketing person mentions 'legacy' in connection with a oldnew service. Especially if it contains other buzzwords like 'SD-WAN' and 'Cloud'. So-
"For example, they can configure Anycast GRE tunnels to connect offices with SD-WAN appliances or basic router hardware, and install the WARP client on employee devices to connect traffic from remote workers.
That's.. Nice. And if all you have is an IP network, then you may already be using GRE tunnels to make an IPVPN. GRE's kinda legacy as well. Advantage MPLS has is it sits between worlds, where bellheads & netheads can meet in an uneasy truce. So slap an 'SD-WAN' appliance (aka router/switch) on the end of an Ethernet and it'll slap small MPLS lables on packets & frames. Core kit, which can be 'cloud', virtualised devices can then switch traffic based on policies applied to VRFs. Those can contain (or be connected) by IP, Ethernet or other transport types. As long as they can read an MPLS header, they can switch/forward traffic. And it's less complex / computationally expensive than routing, and probably Anycasting.
But the challenge for SD-WAN is figuring out how to offer the 'SD' elements, and extend what have traditionally been control-plane functions to the transport layer. And then package it in some way that sales can flog services, customers can understand the benefits, and network engineers don't look terrified.