diff --git a/media/engine/webrtc_video_engine.cc b/media/engine/webrtc_video_engine.cc
index dd91eb2782..b3afbf9fd8 100644
--- a/media/engine/webrtc_video_engine.cc
+++ b/media/engine/webrtc_video_engine.cc
@@ -2502,9 +2502,9 @@ WebRtcVideoChannel::WebRtcVideoSendStream::CreateVideoEncoderConfig(
   // `legacy_scalability_mode` and codec used.
   encoder_config.number_of_streams = parameters_.config.rtp.ssrcs.size();
   bool legacy_scalability_mode = true;
-  // TODO(https://crbug.com/webrtc/14884): This is only used as a kill-switch
-  // in case of serious bugs - when this reaches Stable, delete the field trial.
-  if (!call_->trials().IsDisabled("WebRTC-AllowDisablingLegacyScalability")) {
+  // TODO(https://crbug.com/webrtc/14884): When simulcast VP9 is ready to ship,
+  // don't require a field trial to set `legacy_scalability_mode` to false here.
+  if (call_->trials().IsEnabled("WebRTC-AllowDisablingLegacyScalability")) {
     for (const webrtc::RtpEncodingParameters& encoding :
          rtp_parameters_.encodings) {
       if (encoding.scalability_mode.has_value()) {
diff --git a/media/engine/webrtc_video_engine_unittest.cc b/media/engine/webrtc_video_engine_unittest.cc
index e47f365491..c86ceb6176 100644
--- a/media/engine/webrtc_video_engine_unittest.cc
+++ b/media/engine/webrtc_video_engine_unittest.cc
@@ -8463,9 +8463,7 @@ TEST_F(WebRtcVideoChannelTest,
       webrtc::ScalabilityModeFromString(kDefaultScalabilityModeStr);
   EXPECT_EQ(2, stream->num_encoder_reconfigurations());
   webrtc::VideoEncoderConfig encoder_config = stream->GetEncoderConfig().Copy();
-  // When `scalability_mode` has been specified on any encoding, we do not
-  // reduce the number of simulcast streams.
-  EXPECT_EQ(3u, encoder_config.number_of_streams);
+  EXPECT_EQ(1u, encoder_config.number_of_streams);
   EXPECT_THAT(encoder_config.simulcast_layers,
               ElementsAre(Field(&webrtc::VideoStream::scalability_mode,
                                 ScalabilityMode::kL3T3),
@@ -8478,11 +8476,7 @@ TEST_F(WebRtcVideoChannelTest,
   // VideoStreams are created appropriately for the simulcast case.
   EXPECT_THAT(stream->GetVideoStreams(),
               ElementsAre(Field(&webrtc::VideoStream::scalability_mode,
-                                ScalabilityMode::kL3T3),
-                          Field(&webrtc::VideoStream::scalability_mode,
-                                kDefaultScalabilityMode),
-                          Field(&webrtc::VideoStream::scalability_mode,
-                                kDefaultScalabilityMode)));
+                                ScalabilityMode::kL3T3)));
 
   // GetParameters.
   parameters = send_channel_->GetRtpSendParameters(last_ssrc_);
diff --git a/pc/peer_connection_simulcast_unittest.cc b/pc/peer_connection_simulcast_unittest.cc
index c0513b2a6a..1f5f17a517 100644
--- a/pc/peer_connection_simulcast_unittest.cc
+++ b/pc/peer_connection_simulcast_unittest.cc
@@ -60,6 +60,7 @@
 #include "rtc_base/thread.h"
 #include "rtc_base/unique_id_generator.h"
 #include "system_wrappers/include/metrics.h"
+#include "test/field_trial.h"
 #include "test/gmock.h"
 #include "test/gtest.h"
 
@@ -1431,8 +1432,13 @@ TEST_F(PeerConnectionSimulcastWithMediaFlowTests,
               Optional(std::string("L3T3_KEY")));
 }
 
+// TODO(https://crbug.com/webrtc/14884): A field trial shouldn't be needed to
+// get spec-compliant behavior!
 TEST_F(PeerConnectionSimulcastWithMediaFlowTests,
        SendingThreeEncodings_VP9_Simulcast) {
+  test::ScopedFieldTrials field_trials(
+      "WebRTC-AllowDisablingLegacyScalability/Enabled/");
+
   rtc::scoped_refptr<PeerConnectionTestWrapper> local_pc_wrapper = CreatePc();
   rtc::scoped_refptr<PeerConnectionTestWrapper> remote_pc_wrapper = CreatePc();
   ExchangeIceCandidates(local_pc_wrapper, remote_pc_wrapper);
@@ -1497,9 +1503,17 @@ TEST_F(PeerConnectionSimulcastWithMediaFlowTests,
   EXPECT_THAT(*outbound_rtps[2]->scalability_mode, StrEq("L1T3"));
 }
 
+// TODO(https://crbug.com/webrtc/15005): A field trial shouldn't be needed to
+// get spec-compliant behavior! The same field trial is also used for VP9
+// simulcast (https://crbug.com/webrtc/14884).
 TEST_F(PeerConnectionSimulcastWithMediaFlowTests,
        SendingThreeEncodings_AV1_Simulcast) {
+  test::ScopedFieldTrials field_trials(
+      "WebRTC-AllowDisablingLegacyScalability/Enabled/");
+
   rtc::scoped_refptr<PeerConnectionTestWrapper> local_pc_wrapper = CreatePc();
+  // TODO(https://crbug.com/webrtc/15011): Expand testing support for AV1 or
+  // allow compile time checks so that gates like this isn't needed at runtime.
   if (!HasSenderVideoCodecCapability(local_pc_wrapper, "AV1")) {
     RTC_LOG(LS_WARNING) << "\n***\nAV1 is not available, skipping test.\n***";
     return;