diff --git a/src/systems/abstractsystem.jl b/src/systems/abstractsystem.jl
index 5e9cb0961b..38b2c30e1f 100644
--- a/src/systems/abstractsystem.jl
+++ b/src/systems/abstractsystem.jl
@@ -731,9 +731,6 @@ $(TYPEDSIGNATURES)
 
 Remake the system `sys` with every field replaced by the value in `kwargs`.
 
-If `skip_nonexisting`, fields that do not exist in `sys` are *silently skipped*!
-This means that typos in field names will not error, so use this with caution!
-
 ```julia
 @variables x(t) y(t)
 @named sysx = ODESystem([x ~ 0], t)
@@ -746,7 +743,6 @@ WARNING: intended for internal use; does not perform any sanity checks.
 # TODO: use SciMLBase's generic remake()? doesn't work out of the box, though
 function remake(sys::AbstractSystem; skip_nonexisting = false, kwargs...)
     for (field, value) in kwargs
-        !hasfield(typeof(sys), field) && skip_nonexisting && continue
         # like `Setfield.@set! sys.field = value`, but with `field` replaced by an arbitrarily named symbol
         # (e.g. https://discourse.julialang.org/t/accessing-struct-via-symbol/58809/4)
         sys = Setfield.set(sys, Setfield.PropertyLens{field}(), value)
@@ -2623,27 +2619,19 @@ function extend(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nam
         end
     end
 
-    return remake(sys; skip_nonexisting = true,
-        # fields provided directly to extend()
-        name, gui_metadata,
-
-        # fields common to all system types
-        eqs = union(get_eqs(basesys), get_eqs(sys)),
-        unknowns = union(get_unknowns(basesys), get_unknowns(sys)),
-        ps = union(get_ps(basesys), get_ps(sys)),
-        iv = isempty(ivs) ? missing : ivs[1], # TODO: handle generally
-        parameter_dependencies = union_nothing(get_parameter_dependencies(basesys), get_parameter_dependencies(sys)),
-        observed = union(get_observed(basesys), get_observed(sys)),
-        continuous_events = union(get_continuous_events(basesys), get_continuous_events(sys)),
-        discrete_events = union(get_discrete_events(basesys), get_discrete_events(sys)),
-        defaults = merge(get_defaults(basesys), get_defaults(sys)), # prefer `sys`,
-        systems = union(get_systems(basesys), get_systems(sys)),
-        metadata = union_nothing(get_metadata(basesys), get_metadata(sys)),
-
-        # fields specific to some system types
-        initialization_eqs = hasfield(T, :initialization_eqs) ? union(get_initialization_eqs(basesys), get_initialization_eqs(sys)) : missing,
-        guesses = hasfield(T, :guesses) ? merge(get_guesses(basesys), get_guesses(sys)) : missing, # prefer `sys`
-    )
+    # gracefully fields, being nice if one or both are nothing
+    ext(x, y) = y # prefer sys (y) over basesys (x)
+    ext(x, y::Nothing) = x
+    ext(x::Nothing, y) = ext(y, x)
+    ext(x::Nothing, y::Nothing) = nothing
+    ext(x::AbstractDict, y::AbstractDict) = merge(x, y) # prefer sys (y) over basesys (x)
+    ext(x::AbstractVector, y::AbstractVector) = union(x, y)
+    ext(field::Symbol) = ext(getfield(sys, field), getfield(basesys, field)) # TODO: use get_...?
+
+    # both systems were individually sanity-checked upon construction,
+    # so it should be fine to merge their fields without further checking
+    kwargs = Dict(field => ext(field) for field in fieldnames(T))
+    return remake(sys; kwargs..., name, gui_metadata)
 end
 
 function Base.:(&)(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nameof(sys))
diff --git a/src/utils.jl b/src/utils.jl
index 9aa893e321..52a09bb449 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -1,14 +1,3 @@
-"""
-    union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
-
-Unite x and y gracefully when they could be nothing. If neither is nothing, x and y are united normally. If one is nothing, the other is returned unmodified. If both are nothing, nothing is returned.
-"""
-function union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
-    isnothing(x) && return y # y can be nothing or something
-    isnothing(y) && return x # x can be nothing or something
-    return union(x, y) # both x and y are something and can be united normally
-end
-
 get_iv(D::Differential) = D.x
 
 function make_operation(@nospecialize(op), args)