There are two forms of this argument. Many people don't care about new features but everyone needs to care about security updates and, eventually, a relatively recent browser. If they don't care yet, give them one major exploit and they will next time.
Many Android users have a distinctly inferior web experience because they're running a copy of WebKit from 2011 or so but they'd be perfectly okay with an upgraded browser which has the same UI but supports the more recent features supported by the sites they use. Unlike the app situation, this is more noticeable because sites upgrade outside of your control and Android's fragmentation means that many sites won't make it a major priority when deciding whether to backport or simply disable new features on older devices.
Many Android users have a distinctly inferior web experience because they're running a copy of WebKit from 2011 or so but they'd be perfectly okay with an upgraded browser which has the same UI but supports the more recent features supported by the sites they use. Unlike the app situation, this is more noticeable because sites upgrade outside of your control and Android's fragmentation means that many sites won't make it a major priority when deciding whether to backport or simply disable new features on older devices.